Remove `async_epidata` calls from Epidata and the covidcast Python client

[dshemetov]

See @melange396's comment here for the reasoning behind deprecating this method. After that set of PRs is merged, async_epidata will be deprecated.

This issue is here to track the work of eventually removing this code from both delphi_epidata and from the covidcast Python client. The code in question:

• in the covidcast client here
• in the delphi_epidata client here

[melange396]

SirCAL uses the async functionality and may incur a performance hit when it is removed (it will make its 14 single-day requests sequentially instead of simultaneously). related issue: cmu-delphi/covidcast-indicators#1931