Rename or remove current time_value output column in predictions

[brookslogan]

The current time_value output column indicates the index time_value used to define the test instance for each time series, but that's probably not as useful as (a) it being the target time, so we can easily join back with ground truth data, maybe apply some epi_df plotting tools, etc., or (b) the forecast date, because that'd also probably feel a bit more natural. We should use a more descriptive name or eliminate this column altogether.

[dajmcdon]

closed by #200