Retire GNU_GREP_COMPAT knob

[kevans]

Summary

This was introduced and then disabled by default primarily to avoid dealing with bugs in libgnuregex. rS363823 switched to using libregex for it, so let's just rip the option out now so we can make sure we're getting tested with libregex via bsdgrep.

[emaste]

usr.bin/grep/grep.c

561

can we gc the unused errstr now

[kevans]

usr.bin/grep/grep.c

561

Oh, sure, I'll rip that out, too.