gi: Remove _gi.add_interface()

This used to be a function in the imports._gi module, but it was
rewritten at some point to be called only from C++. The JS function
definition was mistakenly left in. If you tried to call this function
from JS, it would almost certainly crash, so we may as well remove it.

https://bugzilla.gnome.org/show_bug.cgi?id=777205