rc.d: remove obsolete /etc/rc.d/keyserv

the keyserv daemon was removed in 9dcb984251b3. also remove its rc
script.

Reviewed by: kevans, adrian, emaste
Approved by: kevans (mentor)
Fixes: 9dcb984251b3 ("Remove publickey(5) stuff")
Differential Revision: https://reviews.freebsd.org/D50135