um: Remove obsolete legacy network transports

These legacy network transports were marked as obsolete in commit
40814b98 ("um: Mark non-vector net transports as obsolete").
More than five years have passed since then. Remove these network
transports to reduce the maintenance burden.

Suggested-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Link: https://patch.msgid.link/20250503051710.3286595-2-tiwei.btw@antgroup.com


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