Cleanup: fix unused parameter in release builds

Add `UNUSED_VARS_NDEBUG(ac);` to silence the 'unused parameter' warning
in non-debug builds.

No functional changes.

source/blender/editors/animation/anim_channels_edit.cc

@@ -2907,6 +2907,7 @@ static bool animchannels_delete_containers(const bContext *C, bAnimContext *ac)
 
 void ED_anim_ale_fcurve_delete(bAnimContext &ac, bAnimListElem &ale)
 {
+  UNUSED_VARS_NDEBUG(ac);
   BLI_assert(ELEM(ale.type, ANIMTYPE_FCURVE, ANIMTYPE_NLACURVE));
 
   switch (ale.type) {