mapport: Cleanups to port mapping module post UPnP drop

Summary:
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

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19806