um: Remove /proc/sysemu support code

Currently /proc/sysemu will never be registered, as sysemu_supported
is initialized to zero implicitly and no code updates it. And there is
also nothing to configure via sysemu in UML anymore.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20240527134024.1539848-3-tiwei.btw@antgroup.com


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