mingw: remove duplicate `USE_NED_ALLOCATOR` directive

mingw: remove duplicate `USE_NED_ALLOCATOR` directive

nedalloc was added to fix the slowness of memory allocator. Here
specifically for the MSys2 build there seems to be a duplication of
USE_NED_ALLOCATOR directive. So this patch intends to remove the
duplicate USE_NED_ALLOCATOR and keeping it only into the MSys2 config
section so it still uses the nedalloc.

Signed-off-by: Harshil-Jani <harshiljani2002@gmail.com>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>