docs: vfs: clean up after the iterate() removal

Commit 3e327154 ("vfs: get rid of old '->iterate' directory operation")
removed the iterate() file_operations member, but neglected to clean up the
associated documentation.  Get rid of the leftovers.

Link: https://lore.kernel.org/r/874jl945bv.fsf@meer.lwn.net


Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>