[S390] s390: remove DEBUG_MALLOC

The kernel now has kmemleak and kmemtrace so there's no reason to keep
this ugly s390 hack around. I am not sure how it's supposed to work on
SMP anyway as it uses a global variable to temporarily store the return
value of all kmalloc() calls:

  void *b;

  #define kmalloc(x...) (PRINT_INFO(" kmalloc %p\n",b=kmalloc(x)),b)

Cc: <linux-s390@vger.kernel.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>