leavePresence method
Leaves presence in channelName, removing local presence state and sending
a presence leave frame to the server.
Implementation
void leavePresence(String channelName) {
final userInfo = _desiredPresenceJoins.remove(channelName);
_presenceStore.remove(channelName);
if (isConnected && userInfo != null) {
send(RealtimeMessage.presenceLeave(channelName, userInfo));
}
}