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 <sfr@canb.auug.org.au>
Signed-off-by: Jeff Layton <jlayton@kernel.org>

Documentation/filesystems/index.rst

@@ -29,7 +29,6 @@ algorithms work.
    fiemap
    files
    locks
-   mandatory-locking
    mount_api
    quota
    seq_file