midx: remove unused `midx_locate_pack()`

midx: remove unused `midx_locate_pack()`

Commit 307d75bbe6 (midx: implement `midx_locate_pack()`, 2023-12-14)
introduced `midx_locate_pack()`, which was described at the time as a
complement to the function `midx_contains_pack()` which allowed
callers to determine where in the MIDX lexical order a pack appeared, as
opposed to whether or not it was simply contained.

307d75bbe6 suggests that future patches would be added which would
introduce callers for this new function, but none ever were, meaning the
function has gone unused since its introduction.

Clean this up by in effect reverting 307d75bbe6, which removes the
unused functions and inlines its definition back into
`midx_contains_pack()`.

(Looking back through the list archives when 307d75bbe6 was written,
this was in preparation for this[1] patch from back when we had the
concept of "disjoint" packs while developing multi-pack verbatim reuse.
That concept was abandoned before the series was merged, but I never
dropped what would become 307d75bbe6 from the series, leading to the
state prior to this commit).

[1]: https://lore.kernel.org/git/3019738b52ba8cd78ea696a3b800fa91e722eb66.1701198172.git.me@ttaylorr.com/

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