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>