replace the python covidcast client in google-symptoms

[melange396]

similar to #1931 and #1972 , replace the usage of the old covidcast client in the google-symptoms indicator. it is only really used in one place:

covidcast-indicators/google_symptoms/delphi_google_symptoms/run.py

Lines 69 to 70 in 1526dc9

	covidcast.use_api_key(params["indicator"]["api_credentials"])
	metadata = covidcast.metadata()

its a pretty basic usage, and covidcast.metadata() doesnt do a whole lot more than Epidata.covidcast_meta()

[aysim319]

blocked by #1972