Force-closing of proxy connections is broken

[bitphage]

🐞 Describe the bug

#3070 introduced force-closing of proxy connections, though the way it has been implemented (proto.force_close() call) doesn't work. So instead of making such hard-coded things I suggest removing this code and instead adding into documentation something like this: "If you're experiencing timeouts or other issues with proxy connections, use force_close=True to force close proxy connections after each request/response".

💡 To Reproduce

1. Spin up proxy
2. Set up Connector with high keepalive_temeout
3. Run session.get() call in while-loop with asyncio.sleep()
4. watch -n 0.5 'netstat -napt |grep 8888' where 8888 is your proxy port
5. See proxy connections not being closed

💡 Expected behavior

1. No confusing broken code in the library
2. Let the user decide to close or not proxy connections (just use force_close)

📋 Your version of the aiohttp/yarl/multidict distributions

$ python -m pip show aiohttp
Name: aiohttp
Version: 3.6.1
...

📋 Additional context

client