release: stop disabling sendmail

sendmail is already disabled by default everywhere since 14.0

MFC After: 1 week
Approved by: emaste (re)
Reviewed by: emaste (re)
Differential Revision: https://reviews.freebsd.org/D54575

(cherry picked from commit d07198ff72aeddc5a884cfb81b22c22f2abf88f6)