main/sxmo-common: don't disable polkit service in post-{install,upgrade}

rc-update fails if this service is not in the default runlevel,which is the case if it's a brand new installation, or would be the case later on upgrading this package again after the service was already removed from the runlevel.

The issue that c47c62e3 set out to solve isn't fatal, if polkit fails to start by openrc then users can manually remove it from the runlevel.

partially reverts c47c62e3