MOBILE-3720 env: Remove usage of Array.flat

src/core/singletons/array.ts

@@ -37,10 +37,6 @@ export class CoreArray {
      * @return Flattened array.
      */
     static flatten<T>(arr: T[][]): T[] {
-        if ('flat' in arr) {
-            return (arr as any).flat(); // eslint-disable-line @typescript-eslint/no-explicit-any
-        }
-
         return (<T[]> []).concat(...arr);
     }