Update apache-htcacheclean.init to have sensible Default-Start values

With the old value, "systemctl enable apache-htcacheclean" would fail
like this:
 Synchronizing state of apache-htcacheclean.service with SysV service script with /lib/systemd/systemd-sysv-install.
 Executing: /lib/systemd/systemd-sysv-install enable apache-htcacheclean
 update-rc.d: error: apache-htcacheclean Default-Start contains no runlevels, aborting.

So we add meta-data to make the script enabled by default but we actually disable
it immediately in the postinst. And we can clean up some hacks that we introduced
to cover for the Default-Start value being empty.