LinkInterface::bytesAvailable api removal

This is never called because this api doesn’t make any sense given the fact that LinkInterface signals bytesReceived with the bytes in the signal. If bytes are available, then bytesReceived would have already been singled. The data would be in the signal and bytesAvailable would always return 0.

Cleaning my way through the Link stuff as I work my way to a MockLink LinkInterface implementation which will be a configurable simulation of a Pixhawk board. This will be used for better manual testing as well as deeper automated testing.