um: Remove obsolete fixmap support

It was added to support highmem. But since the highmem support has
been removed by commit a98a6d86 ("um: Remove broken highmem
support"), it is no longer needed. Remove it to simplify the code.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20241128081939.2216246-3-tiwei.btw@antgroup.com


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