Remove the use of Pandas' iloc() in WorldMapViz (apache#7379)

When the same metric is used in a World Map panel for both bubble size
and an axis (either X or Y), it currently breaks the rendering.

The change in behavior was introduced in:
apache@71e0c07#diff-f451672348fc6071e8d627778bdc4e96L1730

The use of .iloc() is not needed anymore since the code that used
to duplicate the metric is not there anymore.

Should fix apache#7006

superset/viz.py

@@ -1765,9 +1765,7 @@ def get_data(self, df):
         columns = ['country', 'm1', 'm2']
         if metric == secondary_metric:
             ndf = df[cols]
-            # df[metric] will be a DataFrame
-            # because there are duplicate column names
-            ndf['m1'] = df[metric].iloc[:, 0]
+            ndf['m1'] = df[metric]
             ndf['m2'] = ndf['m1']
         else:
             if secondary_metric: