Retire GNU_GREP_COMPAT knob

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.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D27476