Remove CH-sending timer

CH is an application-level keepalive packet that clients periodically
send for two reasons:

1. It tells the server they're still connected, preventing timeouts.

2. By measuring latency between sending CH and receiving CHECK, a client
   can display ping.

Keepalive is redundant because WebSocket can handle that via PING frames on a
transport layer. WebAO also completely ignores CHECK and sends CH every
five seconds, which is superfluous (AO2 Client sends it once every 45
seconds, in comparison).

Sending CH via `setInterval` was also problematic: browsers seem to
throttle it when the tab becomes inactive, preventing periodic pings and
leading to the server disconnecting inactive browser clients.

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