Stop trying to deal with "theoretical possibilities".

We can't possibly continue to be a faithful GActionGroup implementation
across dispose because dispose has a side effect of removing everyone's
signal handlers.

The code that we ran after the dispose chainup to do all of the fancy
signal emulation was therefore dead.  The test that aimed to verify this
was buggy itself due to an uninitialised variable, so really, it never
worked at all.

We keep the re-ordering of the chainup from the original commit to avoid having
trouble with GtkActionMuxer and keep the checks in place that will prevent an
outright segfault in the case that someone else tries to use the interface
post-dispose.

https://bugzilla.gnome.org/show_bug.cgi?id=722189