Cleanup: Anim: remove declaration of non-existing function

Remove the declaration of `blender::animrig::legacy::action_has_fcurves()`.
Its implementation was never committed to `main`, and the declaration also
shouldn't have.

No functional changes.

source/blender/animrig/ANIM_action_legacy.hh

@@ -34,11 +34,6 @@ ChannelBag *channelbag_get(Action &action);
  */
 ChannelBag &channelbag_ensure(Action &action);
 
-/**
- * Return whether this Action has any F-Curves.
- */
-bool action_has_fcurves(const bAction &action);
-
 /**
  * Return all F-Curves in the Action.
  *