release: Redirect etcupdate logfile to /dev/null.

Stop shipping a log file for etcupdate. This is a source of
non-reproducability as it uses mktemp thereby guaranteeing the output is
different each run.

Differential Revision: https://reviews.freebsd.org/D46317

(cherry picked from commit e972e408d19a58c4f2855eface487d06ef73b799)