test(dialog): remove unused variable from spec file (#26408)

unused variable remove from the test case file

Author: alkavats1

src/cdk/dialog/dialog.spec.ts

@@ -398,7 +398,7 @@ describe('Dialog', () => {
     viewContainerFixture.detectChanges();
     flushMicrotasks();
 
-    dialogRef = dialog.open(PizzaMsg, {
+    dialog.open(PizzaMsg, {
       maxWidth: '100px',
     });