ZFS: Remove a redundant if condition (#12598)

Commit 0c03d21ac99ebdbe left in a redundant if condition while
removing some code. Just remove it.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #12598