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>

Documentation/technical/multi-pack-index.adoc

@@ -167,16 +167,6 @@ m->num_objects_in_base`).
 Future Work
 -----------
 
-- The multi-pack-index allows many packfiles, especially in a context
-  where repacking is expensive (such as a very large repo), or
-  unexpected maintenance time is unacceptable (such as a high-demand
-  build machine). However, the multi-pack-index needs to be rewritten
-  in full every time. We can extend the format to be incremental, so
-  writes are fast. By storing a small "tip" multi-pack-index that
-  points to large "base" MIDX files, we can keep writes fast while
-  still reducing the number of binary searches required for object
-  lookups.
-
 - If the multi-pack-index is extended to store a "stable object order"
   (a function Order(hash) = integer that is constant for a given hash,
   even as the multi-pack-index is updated) then MIDX bitmaps could be