Remove unused `mockBaseClass` helper

The code that used to use this has been refactored to avoid inheritance.

src/test-util/mock-base.js

-/**
- * Mock the base class of a derived class.
- *
- * In unit tests for a derived class it may be useful to mock the base class
- * so that the tests only depend on the interface of the base class and not
- * its implementation.
- *
- * This cannot be done using `$imports.$mock` because the links between the
- * derived and base class are set once when the derived class is initially
- * evaluated.
- *
- * Although it is possible to mock a base class using this function, using
- * composition over inheritance generally makes mocking easier.
- *
- * @param {object} derivedClass - The derived class constructor to mock
- * @param {object} mockBase - The new mock class
- * @return {() => void} A function that un-mocks the base class
- */
-export function mockBaseClass(derivedClass, mockBase) {
-  const originalBase = derivedClass.__proto__;
-
-  // Modify the base class reference used by `super` expressions.
-  derivedClass.__proto__ = mockBase;
-
-  // Modify the prototype chain used by instances of the derived class to find
-  // methods or properties defined on the base class.
-  derivedClass.prototype.__proto__ = mockBase.prototype;
-
-  return () => {
-    derivedClass.__proto__ = originalBase;
-    derivedClass.prototype.__proto__ = originalBase.prototype;
-  };
-}