docs/mm: remove useless markup

docs/mm: remove useless markup

It is enough to use a file name to cross-reference another rst document.

Jon says:
  The right things will happen in the HTML output, readers of the
  plain-text will know immediately where to go, and we don't have to add
  the label clutter.

Drop reference markup and unnecessary labels and use plain file names.

Signed-off-by: Mike Rapoport (IBM) <rppt@kernel.org>
Link: https://lore.kernel.org/r/20230201094156.991542-3-rppt@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>