Documentation: kernel-docs: Remove entry for vfs.txt

It's unnecessary to point to an external mirror of the Documentation
directory.

Jonathan Corbet writes in favor of removing this entry, instead of
moving it under "Docs at the Linux Kernel tree":

> We don't want to turn kernel-docs.rst into yet another out-of-date
> index for the rest of Documentation/, and the removal of the external
> URL takes away the only bit of additional information that this entry
> offers.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>