um: Remove highmem leftovers

Highmem was only supported on UML/i386. And the support has been
removed by commit a98a6d86 ("um: Remove broken highmem support").
Remove the leftovers and stop UML from trying to setup highmem when
the sum of physmem_size and iomem_size exceeds max_physmem.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20240916045950.508910-4-tiwei.btw@antgroup.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>