docs: multi-pack-index: remove note about future 'verify' work

This was implemented in the 'git multi-pack-index' command and
merged in 468b3221 (Merge branch 'ds/multi-pack-verify',
2018-10-10).

And there's no 'git midx' command.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/multi-pack-index.txt

@@ -60,10 +60,6 @@ Design Details
 Future Work
 -----------
 
-- Add a 'verify' subcommand to the 'git midx' builtin to verify the
-  contents of the multi-pack-index file match the offsets listed in
-  the corresponding pack-indexes.
-
 - 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