parisc/slab: cleanup after /proc/slab_allocators removal

The commit 7878c231 ("slab: remove /proc/slab_allocators")
removes DEBUG_SLAB_LEAK config everywhere but a parisc config.
It doesn't look intentional. Fix it.

Signed-off-by: Yury Norov <ynorov@marvell.com>
Signed-off-by: Helge Deller <deller@gmx.de>