refactor(core): remove toString() method from DefaultIterableDiffer

toString() from DefaultIterableDiffer is only used in tests and should not
be part of the production code. toString() methods from differs add
~ 0.3KB (min+gzip) to the production bundle size.

tools/public_api_guard/core/core.d.ts

@@ -345,7 +345,6 @@ export declare class DefaultIterableDiffer<V> implements IterableDiffer<V>, Iter
     forEachPreviousItem(fn: (record: IterableChangeRecord_<V>) => void): void;
     forEachRemovedItem(fn: (record: IterableChangeRecord_<V>) => void): void;
     onDestroy(): void;
-    toString(): string;
 }
 
 /** @experimental */