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>

git-compat-util.h

@@ -1537,7 +1537,6 @@ int cmd_main(int, const char **);
 int common_exit(const char *file, int line, int code);
 #define exit(code) exit(common_exit(__FILE__, __LINE__, (code)))
 
-#define z_const
 #include <zlib.h>
 
 /*