`GSimpleAsyncResult` has been deprecated in favor of the simple GTask
API quite some time ago.

Due to the way `TpMutableContactGroupListInterface` is set up, we now
also have to implement the `_finish` functions (since we don't use the
default GSimpleAsyncResult implementation anymore). Luckily, the
functions have the same signature, so we can just reuse one function
here.