fs: remove NTFS classic from docum. index

With the remove of the NTFS classic filesystem, also remove its
documentation entry from the filesystems index to prevent a
kernel-doc warning:

Documentation/filesystems/index.rst:63: WARNING: toctree contains reference to nonexisting document 'filesystems/ntfs'

Fixes: 9c67092ed339 ("fs: Remove NTFS classic")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20240124011424.731-1-rdunlap@infradead.org
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc:  <linux-fsdevel@vger.kernel.org>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Anton Altaparmakov <anton@tuxera.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc:  <linux-doc@vger.kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>