Remove dead/unreachable assertions in DispatchMapPort, then drop some logic that was used for switching between mapping protocols and is now unneeded.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/31157 | core#31157]] and concludes backport of [[https://github.com/bitcoin/bitcoin/pull/31130 | core#31130]]
https://github.com/bitcoin/bitcoin/pull/31130/changes/40e5f26a3ff77e50df808f6f850c617aec2df203
Depends on D19805