Documentation/gitformat-pack.txt: drop mixed version section

This section was added in 3d89a8c118 (Documentation/technical: add
cruft-packs.txt, 2022-05-20) to highlight a potential pitfall when
deploying cruft packs in an environment where multiple versions of Git
are GC-ing the same repository.

Now that it has been more than a year since 3d89a8c118 was written,
let's drop this section as it is no longer relevant.

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

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