Documentation/gitformat-pack.txt: remove multi-cruft packs alternative

Documentation/gitformat-pack.txt: remove multi-cruft packs alternative

This text, originally from 3d89a8c118 (Documentation/technical: add
cruft-packs.txt, 2022-05-20) lists multiple cruft packs as a potential
alternative to the design of cruft packs.

We have always supported multiple cruft packs (i.e. we use the most
recent mtime for a given object among all cruft packs which contain it,
etc.), but haven't encouraged its use.

We still aren't encouraging users to go out and generate multiple cruft
packs, but let's take a step in that direction by dropping language that
suggests we aren't capable of working with multiple cruft packs.

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