barriers: Fix leak in meta_barrier_destroy

meta_barrier_destroy is responsible for removing the extra
reference added in meta_barrier_constructed.

Unfortunately, it fails to do this because of a misplaced early
return statement.

This commit removes the spurious return.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1449

src/backends/meta-barrier.c

@@ -294,7 +294,7 @@ meta_barrier_destroy (MetaBarrier *barrier)
   MetaBarrierImpl *impl = barrier->priv->impl;
 
   if (impl)
-    return META_BARRIER_IMPL_GET_CLASS (impl)->destroy (impl);
+    META_BARRIER_IMPL_GET_CLASS (impl)->destroy (impl);
 
   g_object_unref (barrier);
 }