util: Don't generate a backtrace on every G_LOG

We may not show the backtrace, but it's prohibitly expensive to generate,
so don't. If someone wants a backtrace they can use the appropriate G_DEBUG
environment variable plus GDB.

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

src/core/display.c

@@ -5332,7 +5332,6 @@ timestamp_too_old (MetaDisplay *display,
       meta_warning ("Got a request to focus %s with a timestamp of 0.  This "
                     "shouldn't happen!\n",
                     window ? window->desc : "the no_focus_window");
-      meta_print_backtrace ();
       *timestamp = meta_display_get_current_time_roundtrip (display);
       return FALSE;
     }