Redundancy: map_addon.py and mitm.py

Currently these two files accomplish the same thing. `map_addon.py` is used when running the mitmproxy directly as a system command, `mitm.py` is a stand-alone script that runs mitmproxy with the same addon functionality baked in.

Need to verify that `mitm.py` functions as expected and remove redundant code

Read map_addon.py and mitm.py side by side, then verify how mitm.py runs mitmproxy and whether its addon behavior matches the system-command path. Confirm the standalone script works as expected, remove the redundant implementation, and verify that the remaining path preserves the same addon functionality.

Written by the indexing model from the issue text.