gio: deprecate GSimpleAsyncResult

GTask has been around for a long time now, everything in GLib is using
it, and the run-in-thread deadlock problems should be fixed now.

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

gio/tests/simple-async-result.c

@@ -14,6 +14,8 @@
 #include <stdlib.h>
 #include <string.h>
 
+G_GNUC_BEGIN_IGNORE_DEPRECATIONS
+
 static GObject      *got_source;
 static GAsyncResult *got_result;
 static gpointer      got_user_data;
@@ -193,3 +195,5 @@ main (int argc, char **argv)
 
   return g_test_run();
 }
+
+G_GNUC_END_IGNORE_DEPRECATIONS