Documentation: remove a "future work" item from the MIDX docs

Documentation: remove a "future work" item from the MIDX docs

One of the items listed as "future work" in the MIDX's technical
documentation is to extend the format to allow MIDXs to be written
incrementally across multiple layers.

This was suggested all the way back in ceab693d1f (multi-pack-index: add
design document, 2018-07-12), and implemented in b9497848df (Merge
branch 'tb/incremental-midx-part-1', 2024-08-19). Let's remove it
accordingly.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>