Remove Suggests dependence on `covidcast`

[dshemetov]

Trying to do that in cmu-delphi/epiprocess#536 and cmu-delphi/epipredict#402. covidcast is incredibly bulky and my renv insists on installing all dependencies including Suggests and it struggles to compile sf (a bulky dependency of covidcast).

There are several reports that sf fails to install as a source package when R is used with renv

From here. Can't find anyone trying to solve this in renv.

Anyway, covidcast is pulled in just to use covidcast::state_census and covidcast::county_census, which are also available as CSVs in that same repo, so we can address this pretty easily by just directly reading them with readr::read_csv. Can probably just copy the code from PRs above.