Deprecate or remove old functions

[cboettig]

There's a handful of functions I think are no longer used, mostly helpers that use the more cumbersome aws.s3 package to download stuff. Is it okay to remove these?

• download_scores.R: (aws.s3 method replaced by much faster method in combined_scores.R)
• download_forecast.R: (read_forecast() can read both nc and csv directly from URL)
• download_target.R: just a wrapper that converted theme names to download urls, maybe not that helpful?
• s3_helpers.R: old aws.s3-based wrappers for downloading, no longer used

I think we also might move noaa.R to a noaa4cast package, since I think that supports use cases that go beyond the neon4cast challenge.

Still core things to keep here:

metadata.R, submit.R validate.R, forecast_output_validator.R fable_helpers.R
(some of these may want to be moved to a more general forecast challenge utilities package?)

[cboettig]

• stack_noaa() / old noaa functions