fix panic with DISCONTIGMEM"

This reverts commit 4a5fa3590f09, which did not allow SLUB to be used
on architectures that use DISCONTIGMEM without compiling NUMA support
without CONFIG_BROKEN also set.

The slub panic that it was intended to prevent is addressed by
d9b41e0b54fd ("[PARISC] set memory ranges in N_NORMAL_MEMORY when
onlined") on parisc so there is no further slub issues with such a
configuration.

The reverts allows SLUB now to be used on such architectures since
there haven't been any reports of additional errors.

Cc: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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