vgabios: remove submodule and build rules.

It's the old, lgpl vgabios implementation.

Was left in as fallback when we switched to seavgabios, so we could
easily switch back in case we see regressions.  It's unused since years
now, reportedly doesn't even build, and lacks support for recently (and
not so recently) added display devices.

Zap it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>