Simplify c2c packets

[xpple]

Big thanks to @wagyourtail. Since we can rely on the sender field of a ClientboundPlayerChatPacket, it is superfluous to include this information in the C2C packet too. This PR excludes that field from transmission.

Because of how this is implemented, listening to C2C packets through /clisten will display the sender field as if it were encoded in the C2C packet. On the one hand this makes sense; the C2C packet classes all have this field. On the other hand it doesn't; it is not actually included in the packet that is transmitted. I'll let others comment on which is better. It could even be made configurable.