action: remove max speed settings

This API is confusing as it only works for PX4 quads but not PX4
fixedwing or anything ArduPilot based.

The reason is that this just sets a PX4 parameter that is not
standardized via MAVLink.

Therefore, I suggest to remove the API and instead recommend to use the
param plugin to set the specific params specifically.

protos/action/action.proto

@@ -137,14 +137,6 @@ service ActionService {
      * Set takeoff altitude (in meters above ground).
      */
     rpc SetTakeoffAltitude(SetTakeoffAltitudeRequest) returns(SetTakeoffAltitudeResponse) {}
-    /*
-     * Get the vehicle maximum speed (in metres/second).
-     */
-    rpc GetMaximumSpeed(GetMaximumSpeedRequest) returns(GetMaximumSpeedResponse) {}
-    /*
-     * Set vehicle maximum speed (in metres/second).
-     */
-    rpc SetMaximumSpeed(SetMaximumSpeedRequest) returns(SetMaximumSpeedResponse) {}
     /*
      * Get the return to launch minimum return altitude (in meters).
      */
@@ -279,19 +271,6 @@ message SetTakeoffAltitudeResponse {
     ActionResult action_result = 1;
 }
 
-message GetMaximumSpeedRequest {}
-message GetMaximumSpeedResponse {
-    ActionResult action_result = 1;
-    float speed = 2; // Maximum speed (in metres/second)
-}
-
-message SetMaximumSpeedRequest {
-    float speed = 1; // Maximum speed (in metres/second)
-}
-message SetMaximumSpeedResponse {
-    ActionResult action_result = 1;
-}
-
 message GetReturnToLaunchAltitudeRequest {}
 message GetReturnToLaunchAltitudeResponse {
     ActionResult action_result = 1;