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Also, the comment above `zfs_readdir()` is out of date.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
3f400b0f58