Avoid systemd_requires as per updated packaging guidelines
There is no need to have them as systemd correctly handles reloading
depending on the transaction ordering (even if systemd is installed
_after_ the package that installs the unit file).
Additionally, this also drops hard requires on /usr/bin/systemctl and
just makes the scriptlets that call systemctl conditional on systemctl
being present.
All of this helps avoid pulling in full systemd for container setups.