Remove FedSockets

Current FedSocket implementation has a bunch of problems. It doesn't have proper error handling (in case of an error the server just doesn't respond until the connection is closed, while the client doesn't match any error messages and just assumes there has been an error after 15s) and the code is full of bad descisions (see: fetch registry which uses uuids for no reason and waits for a response by recursively querying a ets table until the value changes, or double JSON encoding).

Sometime ago I almost completed rewriting fedsockets from scrach to adress these issues. However, while doing so, I realized that fedsockets are just too overkill for what they were trying to accomplish, which is reduce the overhead of federation by not signing every message. This could be done without reimplementing failure states and endpoint logic we already have with HTTP by, for example, using TLS cert auth, or switching to a more performant signature algorithm. I opened #2262 for further discussion on alternatives to fedsockets.

From discussions I had with other Pleroma developers it seems like they would approve the descision to remove them as well, therefore I am submitting this patch.