newvers.sh: Avoid picking up stray envars.

89f361f742ae added a mechanism to allow arbitrary overrides from the
command line. Unfortunately, it also had the (likely unintended)
effect of allowing RELEASE and VERSION to be passed in from the
environment, and Makefile.inc1 happens to define VERSION for the
benefit of pkgbase. To restore the status quo, unset RELEASE and
VERSION at the top of the script.

Fixes: 89f361f742ae
MFC after: 3 days
Reviewed by: gallatin, sjg, emaste
Differential Revision: https://reviews.freebsd.org/D41845