Removing Bitrig support - code review

[Charlotte Koch <charlotte%netbsd.org@localhost>]

Hi!

In pkgsrc-users@ I asked if we should bother supporing Bitrig in pkgsrc
going forward -- and people seemed to agree that there's no point
really.

https://mail-index.netbsd.org/pkgsrc-users/2022/09/21/msg036207.html

So I prepared this diff in the meanwhile, but didn't commit it because
the freeze was on, and also I wanted to make sure I'm on the right
track, and verify that I'm not missing anything:

https://github.com/dressupgeekout/pkgsrc/commit/86125240a5725392edf38da7b4b7f35eb38808dd

What do we think? I'm guessing this change should be documented
somewhere important, too.

Charlotte