fix(widget-builder): Reset legend alias on dataset change (#84194)

The legend alias didn't reset when changing the datasets which didn't
make sense so now it does (for chart widgets). It isn't reset when
changing between chart types (this is the behaviour in the old widget
builder so I'm keeping that). Let me know if there's any other odd
behaviour with the legend aliases that you want me to change i can do it
in this pr.

static/app/views/dashboards/widgetBuilder/hooks/useWidgetBuilderState.spec.tsx

@@ -908,6 +908,33 @@ describe('useWidgetBuilderState', () => {
 
       expect(result.current.state.thresholds).toBeUndefined();
     });
+
+    it('resets the legend alias when the dataset is switched', () => {
+      mockedUsedLocation.mockReturnValue(
+        LocationFixture({
+          query: {
+            dataset: WidgetType.ERRORS,
+            displayType: DisplayType.LINE,
+            legendAlias: ['test'],
+          },
+        })
+      );
+
+      const {result} = renderHook(() => useWidgetBuilderState(), {
+        wrapper: WidgetBuilderProvider,
+      });
+
+      expect(result.current.state.legendAlias).toEqual(['test']);
+
+      act(() => {
+        result.current.dispatch({
+          type: BuilderStateAction.SET_DATASET,
+          payload: WidgetType.TRANSACTIONS,
+        });
+      });
+
+      expect(result.current.state.legendAlias).toEqual([]);
+    });
   });
 
   describe('fields', () => {

static/app/views/dashboards/widgetBuilder/hooks/useWidgetBuilderState.tsx

@@ -288,6 +288,7 @@ function useWidgetBuilderState(): {
 
           setThresholds(undefined);
           setQuery([config.defaultWidgetQuery.conditions]);
+          setLegendAlias([]);
           setSelectedAggregate(undefined);
           break;
         case BuilderStateAction.SET_FIELDS: