Remove old monitor configuration system

Remove the old MetaMonitorConfig system and mark the new one as
non-experimental. This also removes the D-Bus property.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

data/org.gnome.mutter.gschema.xml.in

@@ -103,7 +103,7 @@
     </key>
 
     <key name="experimental-features" type="as">
-      <default>['monitor-config-manager']</default>
+      <default>[]</default>
       <summary>Enable experimental features</summary>
       <description>
         To enable experimental features, add the feature keyword to the list.
@@ -114,22 +114,12 @@
 
         Currently possible keywords:
 
-        • “monitor-config-manager”    — use the new monitor configuration
-                                        system, aimed to replace the old one.
-                                        This enables a higher level configuration
-                                        API to be used by configuration
-                                        applications, as well as the ability to
-                                        configure per logical monitor scale.
         • “scale-monitor-framebuffer” — makes mutter default to layout logical
                                         monitors in a logical pixel coordinate
                                         space, while scaling monitor
                                         framebuffers instead of window content,
                                         to manage HiDPI monitors. Does not
                                         require a restart.
-
-                                        Also enabling “monitor-config-manager”
-                                        is required for this feature to be
-                                        enabled.
       </description>
     </key>