sysctl: update comments to new registration APIs

The sysctl registration APIs do not need a terminating table entry
anymore and with commit acc15469 ("sysctl: Warn on an empty procname
element") even emit warnings if such a sentinel entry is supplied.

While at it also remove the mention of "table->de" which was removed in
commit 3fbfa981 ("[PATCH] sysctl: remove the proc_dir_entry member
for the sysctl tables") back in 2007.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Joel Granados <joel.granados@kernel.org>