logical-monitor: Don't mirror refresh rate

Let the callers figure out how to associate a logical monitor with a
refresh rate, instead of exposing an arbitrary one.

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

src/backends/meta-logical-monitor.c

@@ -43,7 +43,6 @@ meta_logical_monitor_new (MetaMonitor *monitor,
 
   main_output = meta_monitor_get_main_output (monitor);
   logical_monitor->number = number;
-  logical_monitor->refresh_rate = main_output->crtc->current_mode->refresh_rate;
   logical_monitor->winsys_id = main_output->winsys_id;
   logical_monitor->scale = main_output->scale;
   logical_monitor->in_fullscreen = -1;

src/backends/meta-logical-monitor.h

@@ -37,7 +37,6 @@ struct _MetaLogicalMonitor
   int number;
   MetaRectangle rect;
   /* for tiled monitors these are calculated, from untiled just copied */
-  float refresh_rate;
   gboolean is_primary;
   gboolean is_presentation; /* XXX: not yet used */
   gboolean in_fullscreen;