barrier: Make type derivable again

This was used gala to implement hot corners, and the way the barrier API
works, there isn't really any practical reasons to not make it
derivable, since the backend is a separate type and object.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2626>

src/backends/meta-barrier.c

@@ -23,11 +23,6 @@
 #include "backends/native/meta-barrier-native.h"
 #endif
 
-struct _MetaBarrier
-{
-  GObject parent;
-};
-
 typedef struct _MetaBarrierPrivate
 {
   MetaBackend *backend;