Simplify predictions / scoring code

[JedGrabman]

Uses the new version of get_covidhub_predictions by querying multiple forecasters at once.
Also makes use of evaluate_covid_predictions, by evaluating multiple signals at once.

covidcast also now returns actual data for territories, so CovidHubUtils only needs to be used for national level data.

Deletes predictions.R
Refactor score.R

[kateharwood]

This does not adjust the memory usage as far as you know, right? Maybe we should test this with the deploy workflow to make sure everything still runs through?

[JedGrabman]

As far as I know, there shouldn't be a memory impact, but I kicked off the deploy workflow to test it: https://github.com/cmu-delphi/forecast-eval/runs/2222184360?check_suite_focus=true