FreeBSD: Cleanup zfs_readdir()

The FreeBSD project's coverity scans found dead code in zfs_readdir().
Also, the comment above zfs_readdir() is out of date.

I fixed the comment and deleted all of the dead code, plus additional
dead code that was found upon review.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #13924