[feature removal] Remove SockJS transport

[FZambia]

Is your feature request related to a problem? Please describe.

SockJS was deprecated in Centrifugal ecosystem since v4 release.

We asked to reach out if SockJS is still necessary in blog posts, in docs. Nobody reached.

SockJS client is not maintained and some transports becoming archaic.

We now have our own WebSocket emulation layer. It does not require sticky sessions in distributed case, supports binary in HTTP-streaming case, much more performant.

Describe the solution you'd like.

I think it's time to remove SockJS from Centrifugo. The current plan - remove it in Centrifugo v6.