LibJS: Remove String.prototype.length

A string's length property is supposed to be a regular non-writable,
non-enumerable, non-configurable property on the StringObject instead.

Userland/Libraries/LibJS/Tests/builtins/Reflect/Reflect.get.js

@@ -62,6 +62,7 @@ describe("normal behavior", () => {
     });
 
     test("native getter function", () => {
-        expect(Reflect.get(String.prototype, "length", "foo")).toBe(3);
+        const typedArray = new Uint8Array(3);
+        expect(Reflect.get(Uint8Array.prototype, "length", typedArray)).toBe(3);
     });
 });