fix: Disabling timezone of dataframe before passing Prophet

While running forecasting with Druid. Prophet throws the following exception. This PR removes the timezone info. 
ValueError: Column ds has timezone specified, which is not supported. Remove timezone
apache#11106

@villebro

superset/utils/pandas_postprocessing.py

@@ -599,6 +599,7 @@ def _prophet_fit_and_predict(  # pylint: disable=too-many-arguments
         weekly_seasonality=weekly_seasonality,
         daily_seasonality=daily_seasonality,
     )
+    df['ds'] = df['ds'].dt.tz_convert(None)
     model.fit(df)
     future = model.make_future_dataframe(periods=periods, freq=freq)
     forecast = model.predict(future)[["ds", "yhat", "yhat_lower", "yhat_upper"]]