makefs.8: Remove a warning about makefs -t zfs being experimental
This should have been done with commit d9fe71828797 but was missed.
Fixes: d9fe71828797 ("makefs: Remove the warning printed when makefs -t zfs is used")
MFC after: 3 days
(cherry picked from commit e19c607ff9b2b5d55664fd13e5fd9814e3279ffd)