Drop unused county hospitalization aheads early in pipeline instead of in dashboard

[nmdefries]

Hospitalizations forecasts include predictions for 1-28 days ahead, versus the 1-4 weeks ahead used for cases and deaths. However, in the dashboard we actually display only 4 of the 28 aheads for hospitalizations, so we waste a lot of time in the pipeline loading and scoring, and in the dashboard filtering those extra aheads.

The extra aheads were originally included in the pipeline as a "just in case" but haven't been useful yet.