feat(nns): Enable new topics to be followed (#710)

# Why

New topics are created
([context](https://forum.dfinity.org/t/refine-nns-proposals-topics/32125)).
They previously cannot be followed on as NNS Dapp cannot display the
topics yet, but now NNS Dapp can.

# What

Remove the restriction on the mainnet for following to be set on the new
topics.

rs/nns/governance/src/governance.rs

@@ -5833,16 +5833,6 @@ impl Governance {
             )
         })?;
 
-        #[cfg(not(feature = "test"))]
-        if topic == Topic::ProtocolCanisterManagement
-            || topic == Topic::ServiceNervousSystemManagement
-        {
-            return Err(GovernanceError::new_with_message(
-                ErrorType::InvalidCommand,
-                format!("Cannot follow the {:?} topic yet", topic),
-            ));
-        }
-
         self.with_neuron_mut(id, |neuron| {
             if follow_request.followees.is_empty() {
                 neuron.followees.remove(&(topic as i32))

rs/nns/governance/src/governance/tests/mod.rs

@@ -1540,111 +1540,6 @@ fn test_validate_execute_nns_function() {
     }
 }
 
-// TODO(NNS1-3204): Remove this test when the new topics are enabled.
-#[test]
-fn test_follow_new_topics() {
-    // Step 1: set up a neuron with no followees.
-    let mut governance = Governance::new(
-        GovernanceProto {
-            economics: Some(NetworkEconomics::with_default_values()),
-            ..Default::default()
-        },
-        Box::new(MockEnvironment::new(vec![], 100)),
-        Box::new(StubIcpLedger {}),
-        Box::new(StubCMC {}),
-    );
-    let neuron_id = NeuronId { id: 1 };
-    let controller = PrincipalId::new_user_test_id(1);
-    governance
-        .neuron_store
-        .add_neuron(
-            NeuronBuilder::new(
-                neuron_id,
-                Subaccount::try_from(vec![0u8; 32].as_slice()).unwrap(),
-                controller,
-                DissolveStateAndAge::NotDissolving {
-                    dissolve_delay_seconds: 42,
-                    aging_since_timestamp_seconds: 1,
-                },
-                123_456_789,
-            )
-            .build(),
-        )
-        .unwrap();
-
-    // Step 2: sanity check to make sure `follow()` works.
-    governance
-        .follow(
-            &neuron_id,
-            &controller,
-            &manage_neuron::Follow {
-                topic: Topic::Unspecified as i32,
-                followees: [NeuronId { id: 2 }].to_vec(),
-            },
-        )
-        .unwrap();
-
-    // Step 3: following a new topic works with feature = "test".
-    #[cfg(feature = "test")]
-    {
-        assert_eq!(
-            governance.follow(
-                &neuron_id,
-                &controller,
-                &manage_neuron::Follow {
-                    topic: Topic::ProtocolCanisterManagement as i32,
-                    followees: [NeuronId { id: 2 }].to_vec(),
-                },
-            ),
-            Ok(())
-        );
-        assert_eq!(
-            governance.follow(
-                &neuron_id,
-                &controller,
-                &manage_neuron::Follow {
-                    topic: Topic::ServiceNervousSystemManagement as i32,
-                    followees: [NeuronId { id: 2 }].to_vec(),
-                },
-            ),
-            Ok(())
-        );
-    }
-
-    // Step 4: following a new topic fails without feature = "test".
-    #[cfg(not(feature = "test"))]
-    {
-        assert_eq!(
-            governance.follow(
-                &neuron_id,
-                &controller,
-                &manage_neuron::Follow {
-                    topic: Topic::ProtocolCanisterManagement as i32,
-                    followees: [NeuronId { id: 2 }].to_vec(),
-                },
-            ),
-            Err(GovernanceError::new_with_message(
-                ErrorType::InvalidCommand,
-                "Cannot follow the ProtocolCanisterManagement topic yet".to_string()
-            ))
-        );
-        assert_eq!(
-            governance.follow(
-                &neuron_id,
-                &controller,
-                &manage_neuron::Follow {
-                    topic: Topic::ServiceNervousSystemManagement as i32,
-                    followees: [NeuronId { id: 2 }].to_vec(),
-                },
-            ),
-            Err(GovernanceError::new_with_message(
-                ErrorType::InvalidCommand,
-                "Cannot follow the ServiceNervousSystemManagement topic yet".to_string()
-            ))
-        );
-    }
-}
-
 #[test]
 fn topic_min_max_test() {
     use strum::IntoEnumIterator;