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 core#31157 and concludes backport of core#31130
https://github.com/bitcoin/bitcoin/pull/31130/changes/40e5f26a3ff77e50df808f6f850c617aec2df203
Depends on D19805