Retire gnugrep and libgnuregex

[kevans]

Summary

I have this broken into four commits locally, but it should be pretty clear how it's segmented based on the commit messages:

commit 4e363e62433561589730d2197438c7ea15d57531
Author: Kyle Evans <[e-mail]>
Date: Tue Dec 22 15:36:40 2020 -0600

build: remove the option to build gnugrep

Unconditionally install bsdgrep as grep, bootstrap or not. Remove all
build glue and stop installing both gnugrep and libgnuregex now that
all consumers of the latter are gone.

Relnotes: yes

commit bf96e57fb2477fc3ccf05fd996ec3fe5012e14a1
Author: Kyle Evans <[e-mail]>
Date: Tue Dec 22 16:12:49 2020 -0600

src.conf: regenerate after GREP option removal

commit d5823efd7910b63e8d4ca224f956be5fe59ac767
Author: Kyle Evans <[e-mail]>
Date: Tue Dec 22 15:39:12 2020 -0600

grep: tests: stop testing for a nonexistent version of grep

commit b4498d927cbd5568f73b3dbe93a5b19643985404 (HEAD -> kbsd/nogrep)
Author: Kyle Evans <[e-mail]>
Date: Tue Dec 22 15:38:09 2020 -0600

gnu: remove gnugrep and libgnuregex

[bcr]

OK from manpages, too.