test(datetime): remove duplicate test (#28092)

Issue number: N/A

---------

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## What is the current behavior?
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That particular test is flaky. When going to try and fix the flakiness,
I realized that this behavior is already tested by another test. This
test presents the `ion-picker-internal` components which are also tested
in
7c2b6aed05/core/src/components/datetime/test/custom/datetime.e2e.ts (L10).
These show the same components which have the same APIs, so this test
shouldn't even be needed.

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Removed the duplicate test

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->

core/src/components/datetime/test/custom/datetime.e2e.ts

@@ -13,16 +13,6 @@ configs({ directions: ['ltr'] }).forEach(({ title, screenshot, config }) => {
       await expect(datetime).toHaveScreenshot(screenshot(`datetime-custom-wheel`));
     });
 
-    test('should allow styling month/year picker in grid style datetimes', async ({ page }) => {
-      const datetime = page.locator('#custom-grid');
-      const monthYearToggle = datetime.locator('.calendar-month-year');
-
-      await monthYearToggle.click();
-      await page.waitForChanges();
-
-      await expect(datetime).toHaveScreenshot(screenshot(`datetime-custom-month-year`));
-    });
-
     test('should allow styling time picker in grid style datetimes', async ({ page }) => {
       const timeButton = page.locator('ion-datetime .time-body');
       const popover = page.locator('.popover-viewport');