Low: drop legacy SysVStartPriority from service units

Since systemd-218[1], systemd will emit following lines in the journal
upon (re)loading its configuration:

Support for option SysVStartPriority= has been removed and it is ignored

As it allegedly had no importance for some time even before, simply
drop it.

[1] http://cgit.freedesktop.org/systemd/systemd/commit/?id=9e37c95

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>