Remove invalid class macros

Interfaces should not have class cast and type check macros.

The reason why these macros might work, today, is that GTypeInterface and GTypeClass have a similar layout for their first field, and that feeding a GTypeInterface* to a function expecting a GTypeClass* won't likely blow up.

Technically, this is an API break, but not an ABI one. We can punt it to GTK5, if we want to, and at most we're going to useless stuff in the introspection data and/or the documentation.