um: Remove 3-level page table support on i386

The highmem support has been removed by commit a98a6d86 ("um:
Remove broken highmem support"). The 2-level page table is sufficient
on UML/i386 now. Remove the 3-level page table support on UML/i386
which is still marked as experimental.

Suggested-by: Benjamin Berg <benjamin@sipsolutions.net>
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20240918061702.614837-1-tiwei.btw@antgroup.com


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