awk: Remove last markings we have on awk

We normally don't add $FreeBSD$ to contrib software. However, these
changes date back to the CVS era of source code management and have been
overlooked. Now that all these files are back to the same as the
upstream bsd-features branch, remove the FreeBSD specific changes, which
are now just $FreeBSD$ and the (FreeBSD) in the version string.

MFC After: 2 weeks
Sponsored by: Netflix

(cherry picked from commit 5ab82b00cca74feb9d047e9c8aa29e5118810edc)