makefs: Remove the warning printed when makefs -t zfs is used

We haven't seen bug reports relating to this feature for a while, so
stop printing a warning.

Approved by: re (kib)
Reviewed by: cperciva
MFC after: 2 weeks

(cherry picked from commit d9fe71828797508d1d28655e40780a5ae9078e66)
(cherry picked from commit ef89eefaa86f315feba6c44f2059582a077a78ed)