eth: remove EventMux accessors

[darioush]

Hi, it seems these methods in the backend.go and api_backend.go files are not used that expose the eventMux, but that is not needed.

[karalabe]

I think that event mux was something we wanted dot get rid of altogether from everywhere. Any particular place that still references which we could rework to nuke out further?

[darioush]

It is used in eth/downloader to signal the start of the sync to the downloader API, and in cmd/geth/main.go to signal the end of the sync in case ExitWhenSyncedFlag is set. (see DoneEvent)

[fjl]

We could convert these events to be sent using event.FeedOf. That wasn't really possible earlier because we had the miner->downloader cross dependency, but should be doable now.