gtype: remove interface-after-init exceptions

A year ago, we tried to remove support for adding interfaces on
already-initialised types.  There were problems with the C++ and C#
bindings at the time, so we added exceptions to give them a bit more
time to catch up.

It's already one cycle after when these exceptions were planned to be
removed, so let's take them out now.

https://bugzilla.gnome.org/show_bug.cgi?id=697229