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/main.c

@@ -110,7 +110,6 @@ log_handler (const gchar   *log_domain,
              gpointer       user_data)
 {
   meta_warning ("Log level %d: %s\n", log_level, message);
-  meta_print_backtrace ();
 }
 
 /**