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Also, the comment above `zfs_readdir()` is out of date.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

include/os/freebsd/Makefile.am

@@ -21,7 +21,6 @@ noinst_HEADERS = \
 	%D%/spl/sys/dirent.h \
 	%D%/spl/sys/disp.h \
 	%D%/spl/sys/dkio.h \
-	%D%/spl/sys/extdirent.h \
 	%D%/spl/sys/fcntl.h \
 	%D%/spl/sys/file.h \
 	%D%/spl/sys/freebsd_rwlock.h \