Delete unused enable_key_gesture_handler_for_recents flag

1. This flag is replaced by a read only flag, grant_manage_key_gestures_to_recents.
2. This flag is only ever in development. It is therefore safe to delete.

Flag: EXEMPTED remove unused flag
Test: None
Bug: 406452076
Change-Id: I7824c379ba94a054a9697dd995e6d1d38a6a67a8