Remove WeakMap/WeakSet#clear method #6500

// lib.es6.d.ts and lib.core.es6.d.ts
// also WeakSet
 interface WeakMap<K, V> {
-    clear(): void;
     delete(key: K): boolean;
     get(key: K): V;
     has(key: K): boolean;
     set(key: K, value?: V): WeakMap<K, V>;
     [Symbol.toStringTag]: "WeakMap";
 }