Remove netplay code

[r2dliu]

We don't need any of the built in netplay code or any of the ui related to dolphins built in netplay, since we never use them. However, we do store some slippi related variables in those files...move them elsewhere and then delete netplay related code.

[bemug]

Is it really necessary ? I'll be happy to use the Slippi Dolphin build to play online with friends even without slippi rollback.

For example if I want to play as 3 players, play against CPUs, etc.