Remove warning in g_simple_async_result_complete

Remove warning in g_simple_async_result_complete

This warning hits code that uses GSimpleAsyncResult outside of a
mainloop as a helper object. For instance EggDBus does this.
Since the bugs this warning would fix are pretty easy to spot
and since EggDBus is deployed already we just remove the
"called from outside main loop" warning.

However, we need to keep the "called from wrong context" warning
as that is very helpful when debugging misuse of the new multiple
main context code.