monitor-manager: Move PNP lookup to MetaBackend

It's not really about monitors, even though it is used for monitors.
Lets shrink MetaMonitorManager a bit moving it to the backend.

While at it, stop leaking it too.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2141>

src/backends/meta-monitor-manager-private.h

@@ -160,10 +160,6 @@ struct _MetaMonitorManager
 
   MetaMonitorConfigManager *config_manager;
 
-#ifdef HAVE_GNOME_DESKTOP
-  GnomePnpIds *pnp_ids;
-#endif
-
   MetaMonitorSwitchConfigType current_switch_config;
 
   MetaPrivacyScreenChangeState privacy_screen_change_state;
@@ -430,9 +426,6 @@ void meta_monitor_manager_clear_crtc (MetaCrtc *crtc);
 
 gboolean meta_monitor_has_aspect_as_size (MetaMonitor *monitor);
 
-char * meta_monitor_manager_get_vendor_name (MetaMonitorManager *manager,
-                                             const char         *vendor);
-
 static inline MetaOutputAssignment *
 meta_find_output_assignment (MetaOutputAssignment **outputs,
                              unsigned int           n_outputs,