contrib: dracut: parse-zfs: stop pretending we support FILESYSTEM=

It was added in the original ae26d0465a ("Add dracut support") commit
in 2011, and was then broken a bit later with the advent of
dracut-zfs-generator, or maybe earlier as part of other churn

Either way, it's broken, and has been in 2.0+ as well, and no-one
complained. Stop pretending we support it at all

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13291