git-compat-util: drop `z_const` define

Before including <zlib.h> we explicitly define `z_const` to an empty value. This has the effect that the `z_const` macro in "zconf.h" itself will remain empty instead of being defined as `const`, which effectively adapts a couple of APIs so that their parameters are not marked as being constants. It is dubious though whether this is something we actually want: not marking a parameter as a constant doesn't make it any less constant than it was. The define was added via 07564773c2 (compat: auto-detect if zlib has uncompress2(), 2022-01-24), where it was seemingly carried over from our internal compatibility shim for `uncompress2()` that was removed in the preceding commit. The commit message doesn't mention why we carry over the define and make it public, either, and I cannot think of any reason for why we would want to have it. Drop the define. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>