meta-plugin-manager: Only allow one plugin to be loaded

The "multiple plugins loaded at once" strategy was always a big fiction:
while it may be viable if you're super careful, it's fragile and requires
a bit of infrastructure that we would be better off without.

Note that for simplicity, we're keeping the MetaPluginManager, but it only
manages one plugin. A possible future cleanup would be to remove it entirely.

https://bugzilla.gnome.org/show_bug.cgi?id=676855

src/core/mutter.c

@@ -80,11 +80,7 @@ main (int argc, char **argv)
     }
 
   if (plugin)
-    {
-      MetaPluginManager *mgr;
-      mgr = meta_plugin_manager_get_default ();
-      meta_plugin_manager_load (mgr, plugin);
-    }
+    meta_plugin_manager_load (plugin);
 
   meta_init ();
   return meta_run ();