Consider deprecation/removal of delphi_epidata.R client

[melange396]

Reasons to remove, deprecate, or at least just stop supporting the delphi_epidata R client:

• it is superseded by the actively- and well-maintained epidatr package
  • even the covidcast R client has been documented as deprecated in favor of epidatr, and the covidcast R client has been better maintained than this delphi_epidata R client; compare:
    • https://github.com/cmu-delphi/covidcast/commits/main/R-packages
    • https://github.com/cmu-delphi/delphi-epidata/commits/dev/src/client/delphi_epidata.R
• its presence adds dependencies and slows the build+test process while adding to docker image sizes
  • this can improve the development cycle, in the spirit of cruft removal attempt #1308 and Dependency cleanup #1372
• it is fairly safe to do so because:
  • it doesn't provide backing (ie, serve as a dependency) for the covidcast R client (unlike the covidcast python client, which is built upon the delphi_epidata python client)
  • the only way anyone uses this client is if they pull it directly out of this git repo (as opposed to epidatr and the covidcast r client which are published on cran), which seems unlikely/rare
    • according to logs, we've only had 5 non-affiliated users total (none of which were api keyed) in the past 8 months (since we made this client's user agent string identifiable).
• the changeset is small:
  • one file for the client
  • one file for its tests
  • some r installation commands in a Dockerfile (this is the main thing i would like to remove (to speed up builds))
  • a directive for running the tests in a Makefile (which becomes useless if we remove the above commands from the Dockerfile)

[dshemetov]

I've never used this client so it would not affect me. Seems safe, really determined users can just switch to pulling this old client out of Git history, but the majority have likely switched away by now.