Revert "Federation outbound proxy" (#15910)

changelog.d/15773.feature

docs/usage/configuration/config_documentation.md

docs/workers.md

synapse/app/_base.py

synapse/app/generic_worker.py

synapse/app/homeserver.py

synapse/config/workers.py

synapse/http/client.py

synapse/http/matrixfederationclient.py

synapse/http/proxy.py

synapse/http/proxyagent.py

synapse/http/server.py

synapse/http/site.py

tests/app/test_openid_listener.py

tests/handlers/test_device.py

tests/handlers/test_federation.py

tests/handlers/test_presence.py

tests/handlers/test_typing.py

tests/http/test_matrixfederationclient.py

tests/http/test_proxy.py