Remove empty [on_redo] action

This was introduced this development cycle (1.13) in 2015 (commit 6010455f56)
and removed in 33c2e6aa67. Presumably the action was left for backwards
compatibility, but we do not guarantee backwards compatibility for changes that never made it to a stable
release.

src/game_events/action_wml.cpp

@@ -970,9 +970,5 @@ WML_HANDLER_FUNCTION(on_undo, event_info, cfg)
 	}
 }
 
-WML_HANDLER_FUNCTION(on_redo, , )
-{
-}
-
 } // end namespace game_events