Documentation: remove reference to now removed mandatory-locking doc

When I removed the document on mandatory locking from the tree, I missed
removing the reference to it from the index file.

Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Jeff Layton <[e-mail]>

Documentation/filesystems/index.rst

···
    fiemap
    files
    locks
-   mandatory-locking
    mount_api
    quota
    seq_file