Remove unused 'chunkVersion' field sent in splitChunk command

SERVER-27510 made splitChunk stop using the ChunkVersion sent in the 'chunkVersion' field of the command. This field should therefore stop being sent, since it's no longer used.

Note: splitChunk also has another ChunkVersion field, 'shardVersion', from which the epoch is extracted for comparison to ensure the collection hasn't been dropped and re-created. See here.