removing the agp(4) driver

I'd like to propose removing the agp(4) driver in FreeBSD 15.0.  It is
Giant-locked, so during attach already prints a warning about removal.
Moreover, while trying to fix a problem that was triggering panics
during agp attach, I inadvertently broke the device file interface in
14.0, which appears to have gone unreported until quite recently, see
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=281035

From this I infer that agp is effectively unused, and it's certainly
unmaintained.  Given that and the above, let's remove it:
https://reviews.freebsd.org/D46542
https://reviews.freebsd.org/D46543

Feedback or objections are welcome here or on the reviews.