* Remove mostly unused code from FIRMessagingLogger
This cleans up some left-over old logging logic from before we moved to FIRLogger in Firebase Core. We no longer need this logging functionality.
* Delete FIRMessagingConfig.{h,m}
We no longer need a config class to store an unused log level filter (that filter is no longer needed).
This shows the simple property, shouldEstablishDirectChannel, to open a new channel, and how to show incoming messages with the iOS 10 delegate handler