git-compat-util.h: drop the `PRIuMAX` and other fallback definitions

git-compat-util.h: drop the `PRIuMAX` and other fallback definitions

Git's code base already seems to be using `PRIdMAX` without any such
fallback definition for quite a while (75459410edd (json_writer: new
routines to create JSON data, 2018-07-13), to be precise, and the
first Git version to include that commit was v2.19.0).  Having a
fallback definition only for `PRIuMAX` is a bit inconsistent.

We do sometimes get portability reports more than a year after the
problem was introduced.  This one should be fairly safe.  PRIuMAX is
in C99 (for that matter, SCNuMAX, PRIu32 and others also are), and
we've been picking up other C99-isms without complaint.

The PRIuMAX fallback definition was originally added in 3efb1f343a
(Check for PRIuMAX rather than NO_C99_FORMAT in fast-import.c.,
2007-02-20). But it was replacing a construct that was introduced in
an even earlier commit, 579d1fbfaf (Add NO_C99_FORMAT to support
older compilers., 2006-07-30), which talks about gcc 2.95.

That's pretty ancient at this point.

Signed-off-by: Hariom Verma <hariom18599@gmail.com>
Helped-by: Jeff King <peff@peff.net>
[jc: tweaked both message and code, taking what peff wrote]
Signed-off-by: Junio C Hamano <gitster@pobox.com>