devel/cvs: remove libgnuregex dependency.

The only usage of regular expression in cvs was in import.c, and for
that particular usage, no GNU extension was used; with the removal of
libgnuregex, we could just drop the dependency and use the base system
regex(3) instead.

This fixes build on -CURRENT.

PR: 252248
Approved by: maintainer
MFH: 2021Q1