git-multi-pack-index(1): remove non-existent incompatibility

git-multi-pack-index(1): remove non-existent incompatibility

Since fcb2205b774 (midx: implement support for writing incremental MIDX
chains, 2024-08-06), the command-line options '--incremental' and
'--bitmap' were declared to be incompatible with one another when
running 'git multi-pack-index write'.

However, since 27afc272c49 (midx: implement writing incremental MIDX
bitmaps, 2025-03-20), that incompatibility no longer exists, despite the
documentation saying so. Correct this by removing the stale reference to
their incompatibility.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>