loader: Remove gfx_fb_stub.c, it's no longer needed

Now that we draw in the gfx bindings for all our interpreters only when
graphics support is compiled in, we can eliminate this from all the
loaders that don't have graphics support.

Sponsored by: Netflix
Reviewed by: kevans, jhb
Differential Revision: https://reviews.freebsd.org/D43905

(cherry picked from commit 23d9b5c9febf4b02957d01bc46ee75530b0dcb4a)