pkgbase: Remove FreeBSD-libregex package

The only user of libregex is grep (and its variation), no need for a
dedicated package.
This moves libregex to the default package (FreeBSD-utilities).

Differential Revision: https://reviews.freebsd.org/D31802

(cherry picked from commit dfa9131d709121b2e502a82ff66cf3e376654942)