mingw: remove the vendored compat/nedmalloc/ subtree

mingw: remove the vendored compat/nedmalloc/ subtree

The previous two commits stopped opting into nedmalloc on Windows
and stripped out the build-system plumbing that referenced it; the
compat/nedmalloc/ subtree now has no callers and no consumers in
the build, so retire it from the tree.

Note that this patch is larger than can be sent via the mailing
list, and was originally sent in three-pieces and merged back on the
receiving end.

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