Remove producesSortedKeys from BoundedSource

This is a holdover from a precursor of the old Dataflow SDK that we just failed to delete before releasing Dataflow 1.0, but we can delete before the first stable release of Beam.

This function has never been used by any runner. It does not mean anything obvious to implementors, as many sources produce T, not KV<K,V> – what does it mean in the former case? (And how do you get the latter case correct?)