mm: remove __krealloc

Since 5.5-rc1 the last user of this function is gone, so remove the
functionality.

See commit
2ad9d774

 ("netfilter: conntrack: free extension area immediately")
for details.

Link: http://lkml.kernel.org/r/20191212223442.22141-1-fw@strlen.de
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>