Remove validation protocol versions 1 and 2

[alindima]

Validation protocol 2 was built for async backing and validation protocol 3 was built for approval voting improvements.
Both features have been enabled via runtime once sufficient number of validators have upgraded.
Data shows almost all validators (1 or 2 remaining on kusama) have upgraded and are using the latest validation network protocol.

We can therefore remove explicit code that is handling the old versions and refuse new connections on them. This allows us to cleanup unused code and possibly do some local simplifications in network subsystems.