mingw: stop using nedmalloc

mingw: stop using nedmalloc

The vendored nedmalloc allocator under compat/nedmalloc/ has been
unmaintained upstream for a very long time: the original repository at
https://github.com/ned14/nedmalloc received its last commit on July 5,
2014, and was archived (made read-only) by its owner on March 15, 2019.
Our copy has been carried forward unchanged ever since.

The Git for Windows commit that introduced mimalloc as a replacement
on Windows ("mingw: use mimalloc", 2019-06-24, present in the Git for
Windows branch thicket but not upstream) already observed at that time
that nedmalloc had ceased to see any updates for several years.

This came to a head when the Git for Windows SDK upgraded to GCC 16:
the `add_segment()` function in `compat/nedmalloc/malloc.c.h` declares
`int nfences = 0` and only references it inside an `assert()`, which
GCC 16 now flags as `-Wunused-but-set-variable`. Combined with the
`-Werror` enabled by `DEVELOPER=1`, this turns into a hard build
failure:

	compat/nedmalloc/malloc.c.h: In function 'add_segment':
	compat/nedmalloc/malloc.c.h:3897:7: error: variable 'nfences' set but not used [-Werror=unused-but-set-variable=]
	 3897 |   int nfences = 0;
	      |       ^~~~~~~
	cc1.exe: all warnings being treated as errors

The same source built without complaint under GCC 15.2.0; the
regression was bisected to the SDK package update at
https://github.com/git-for-windows/git-sdk-64/commit/188d93dd455
(`mingw-w64-x86_64-gcc 15.2.0-14 -> 16.1.0-1`), with the failing CI
run captured at
https://github.com/git-for-windows/git-sdk-64/actions/runs/25244795074.

Rather than patch the unmaintained vendored sources to silence the
warning, stop opting into nedmalloc altogether on Windows. The
platform allocator is what every non-MINGW build already uses, and a
fresh build of git.git's master against a minimal Git for Windows SDK
upgraded to GCC 16 completes successfully.

The compat/nedmalloc/ subtree itself is removed by subsequent commits
in this series.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>