[EDR Worfklows][FF Cleanup] Remove eventCollectionDataReductionBannerEnabled feature flag (elastic#239576)

## Summary

This PR removes the `eventCollectionDataReductionBannerEnabled` feature
flag, which has been enabled by default. The event collection data
reduction banner functionality is now permanently available.

## Changes

### Source Files

**policy_settings_form.tsx**
- Removed
`useIsExperimentalFeatureEnabled('eventCollectionDataReductionBannerEnabled')`
hook call
- Simplified banner visibility logic to only rely on browser storage
(`securitySolution.showEventMergingBanner`)
- The Event Merging Banner now always shows by default on first visit
and can be permanently dismissed by users

### Test Files

**policy_settings_form.test.tsx**
- Replaced feature flag mocks with storage mocks for banner visibility
control
- Removed obsolete test case for feature flag disabled state
- Updated test setup to use `storageMock.set()` instead of
`setExperimentalFlag()`

### Configuration

**experimental_features.ts**
- Removed `eventCollectionDataReductionBannerEnabled` flag definition

## Behavior After This Change

- The Event Merging Banner will be shown by default to all users who
haven't previously dismissed it
- Users can dismiss the banner, and this preference is stored in browser
storage
- No feature flag check is performed - the feature is always enabled

Author: szwarckonrad

x-pack/solutions/security/plugins/security_solution/common/experimental_features.ts

@@ -200,11 +200,6 @@ export const allowedExperimentalValues = Object.freeze({
    */
   responseActionsMSDefenderEndpointEnabled: true,
 
-  /**
-   * Enables banner for informing users about changes in data collection.
-   */
-  eventCollectionDataReductionBannerEnabled: true,
-
   /** Enables new Data View Picker */
   newDataViewPickerEnabled: true,
 

x-pack/solutions/security/plugins/security_solution/public/management/pages/policy/view/policy_settings_form/policy_settings_form.test.tsx

@@ -51,23 +51,16 @@ describe('Endpoint Policy Settings Form', () => {
       'data-test-subj': 'test',
     };
 
-    mockedContext.setExperimentalFlag({ eventCollectionDataReductionBannerEnabled: false });
+    storageMock.set('securitySolution.showEventMergingBanner', false);
 
     render = () => (renderResult = mockedContext.render(<PolicySettingsForm {...formProps} />));
   });
 
   describe('event merging banner', () => {
     beforeEach(() => {
-      mockedContext.setExperimentalFlag({ eventCollectionDataReductionBannerEnabled: true });
+      storageMock.set('securitySolution.showEventMergingBanner', true);
     });
 
-    it('should hide the banner if its not allowed to be displayed', () => {
-      mockedContext.setExperimentalFlag({ eventCollectionDataReductionBannerEnabled: false });
-
-      render();
-
-      expect(renderResult.queryByTestId('eventMergingCallout')).not.toBeInTheDocument();
-    });
     it('should show the event merging banner if it has never been dismissed', () => {
       render();
 

x-pack/solutions/security/plugins/security_solution/public/management/pages/policy/view/policy_settings_form/policy_settings_form.tsx

@@ -49,9 +49,6 @@ export const PolicySettingsForm = memo<PolicySettingsFormProps>((props) => {
 
   const { storage } = useKibana().services;
 
-  const eventCollectionDataReductionBannerEnabled = useIsExperimentalFeatureEnabled(
-    'eventCollectionDataReductionBannerEnabled'
-  );
   const trustedDevices = useIsExperimentalFeatureEnabled('trustedDevices');
 
   const renderDeviceControlSection = () => {
@@ -72,8 +69,7 @@ export const PolicySettingsForm = memo<PolicySettingsFormProps>((props) => {
   };
 
   const [showEventMergingBanner, setShowEventMergingBanner] = useState(
-    eventCollectionDataReductionBannerEnabled &&
-      (storage.get('securitySolution.showEventMergingBanner') ?? true)
+    storage.get('securitySolution.showEventMergingBanner') ?? true
   );
   const onBannerDismiss = useCallback(() => {
     setShowEventMergingBanner(false);