um: Remove legacy network transport infrastructure

All legacy network transports have been removed. Vector transports
provide the same capabilities with significantly higher network
throughput. There is no reason to keep the legacy network transport
infrastructure anymore. Remove it to reduce the maintenance burden.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Link: https://patch.msgid.link/20250503051710.3286595-4-tiwei.btw@antgroup.com


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