Remove unused iov_iter_init_compat() wrapper

This compatibility code is no longer needed. For it a while
iov_iter_init_compat() was used by zfs_uio_prefaultpages() but
this code should have been dropped as part of commit 83b91ae1.
Take care of that oversight and remove it.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11543