Update issues&lags exclusivity checks to also consider `as_of`

[brookslogan]

I believe covidcast no longer supports as_of together with issues or lags. Other endpoints might still support. We should update the client-side checks. I believe it's also in an Issue somewhere to detect this on the server side as well.

[dshemetov]

We probably don't need to do this in the client, since we can forward an informative error from the server once it's implemented.

[dshemetov]

Update: looks like the server doesn't do validation either, so we should add some guardrails after all cmu-delphi/delphi-epidata#1159

[brookslogan]

Doing checks (a) [server-side and forwarding error in client] or (b) both client-side and server-side both make sense. Currently we have bad checks both client-side and server-side, so both would need updated for either of the above two approaches.

[dshemetov]

Update: from the discussion in the linked thread above, I'm now leaning away from doing these validation checks and actually removing the existing issue/lag checks.