bsdgrep: disable TRE implementation by default

Start the phasing out of TRE by disabling it by default. r317254 introduced
a BSD_GREP_FASTMATCH knob (defaulting to on) for testing of bsdgrep with and
without TRE enabled. More bugs have cropped up since then, and
WITHOUT_BSD_GREP_FASTMATCH has shown in testing to be more stable than its
counterpart.

Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D12381