refactor(derive): drop tokio dependency

The references to 'tokio' in `async-dropper-derive` are in quoted
code, that crate is a proc macro that doesn't need tokio to run
(ie. to manipulate token streams).

Leaving it in as a dependency just means tokio would show up twice
in the dep graph, built once for `async-dropper-derive` (as a proc
macro dependency and unused), and once as a regular dependency.

Having it as a dependency of `async-dropper-derive` also doesn't
force users of the `derive` feature to have a dependency on tokio:
they can still have compile errors due to the `tokio` symbol not
being found if they don't _also_ depend on tokio in their crate,
just like how <https://docs.rs/tracing-test/latest/tracing_test/>
cannot force your crate to depend on `tracing` (even though its
macro expansion needs it).

crates/async-dropper-derive/Cargo.toml

@@ -17,7 +17,7 @@ proc-macro = true
 
 [features]
 default = []
-tokio = ["dep:tokio"]
+tokio = []
 async-std = ["dep:async-std"]
 
 [dependencies]
@@ -26,7 +26,6 @@ async-trait.workspace = true
 proc-macro2.workspace = true
 quote.workspace = true
 syn.workspace = true
-tokio = { workspace = true, optional = true }
 
 [package.metadata.docs.rs]
 all-features = false