Remove abort channel between Ethereum oracle and `geth` subprocess

[james-chf]

This PR removes logic to do with having a channel from the Ethereum oracle task to the geth task so that geth could be shut down if the Ethereum oracle (unexpectedly) terminated. Shutting down of both of these components is now managed from above by AbortableSpawner. If either one terminates for some reason (or any other task like Tendermint, the ABCI server, etc.), then both will get shut down by the AbortableSpawner in any case.