gthreadpool: set default max_unused_threads and max_idle_time values

GThreadPool defaulted to 0 for max_unused_threads (meaning thread-pool
threads would exit immediately if there was not already another task
waiting for them), and 0 for max_idle_time (meaning unused threads
would linger forever, though this is only relevant if you changed
max_unused_threads).

However, GIOScheduler changed the global defaults to 2 and 15*1000,
respectively, arguing that these were more useful defaults. And they
are, so let's use them.

https://bugzilla.gnome.org/show_bug.cgi?id=661767

gio/gioscheduler.c

@@ -113,12 +113,6 @@ init_scheduler (gpointer arg)
 	  g_thread_pool_set_sort_function (job_thread_pool,
 					   g_io_job_compare,
 					   NULL);
-	  /* It's kinda weird that this is a global setting
-	   * instead of per threadpool. However, we really
-	   * want to cache some threads, but not keep around
-	   * those threads forever. */
-	  g_thread_pool_set_max_idle_time (15 * 1000);
-	  g_thread_pool_set_max_unused_threads (2);
 	}
     }
   return NULL;