rpm: Don't attempt to restart on upgrade when using systemd
systemd is actually rather capable of leaving the OpenAFS client in an
incredibly broken state, thanks to its willingness to track services and
kill their processes. We should not attempt to restart the client on
upgrade, whether a normal upgrade or a migration from SysV initscripts.
In the former case, it's fine (and correct) for the old AFS to keep
running; in the latter case, the unit file is capable of correctly
shutting down an initscript-launched client. The same is true for the
OpenAFS server.
This brings the packaging in line with the SysV initscript code in the
specfile, which does not attempt to restart the service, as well as with
e.g. Debian's packaging, which uses --no-restart-on-upgrade.
While we're here, clean up a redundant BuildRequires on systemd-units.