Gio::ActionGroup: Remove a nonsensical check.

* gio/src/actiongroup.hg: get_action_state_hint():
  Remove a wrongly copy/pasted g_return_if_fail() line that
  causes a compiler error when the template is actually used.
  Ideally we would have tests for uses of templated methods.
  Bug #709963 (Johannes Meng, David Evans)