influxdb - fix nil pointer usage - fixes #100723 (#100724)

When introducing errorsource over in:

https://github.com/grafana/grafana/pull/99900

I introduced a bug - trying to use a http response with a non-nil error.
In that case, the response is nil, so code panics.

This PR removes that check.

pkg/tsdb/influxdb/influxql/influxql.go

@@ -177,7 +177,6 @@ func execute(ctx context.Context, tracer trace.Tracer, dsInfo *models.Datasource
 	if err != nil {
 		return backend.DataResponse{
 			Error: err,
-			ErrorSource: backend.ErrorSourceFromHTTPStatus(res.StatusCode),
 		}, err
 	}
 	defer func() {