fix-next(iOS): set root viewe css classes before setup UI (#8005)

At application launch, `ns-root` class is not set when loading the theme.

nativescript-core/css/system-classes.ts

@@ -30,8 +30,3 @@ export function removeFromRootViewCssClasses(value: string): string {
 
     return removedElement;
 }
-
-export function resetRootViewCssClasses(): string[] {
-    // Preserve the default `ns-root` CSS class
-    return rootViewCssClasses.splice(1);
-}