Remove an unusued member from MainContext

The MainContext structure has a field for the master clock, but
clutter_master_clock_get_default() will return the same instance
from everywhere.

clutter/clutter-main.c

@@ -1064,8 +1064,6 @@ clutter_context_get_default (void)
       ctx->is_initialized = FALSE;
       ctx->motion_events_per_actor = TRUE;
 
-      ctx->master_clock = _clutter_master_clock_get_default ();
-
 #ifdef CLUTTER_ENABLE_DEBUG
       ctx->timer          =  g_timer_new ();
       g_timer_start (ctx->timer);

clutter/clutter-private.h

@@ -128,7 +128,6 @@ struct _ClutterMainContext
 
   guint32 last_event_time;
 
-  ClutterMasterClock *master_clock;
   gulong redraw_count;
 
   GList *repaint_funcs;