[cleanup] Remove unused package jest-mock-scheduler (#26084)

## Summary

Removing package jest-mock-scheduler introduced in PR
https://github.com/facebook/react/pull/14358, as it is no longer
referenced in the main branch code. The following files previously
referenced it:

- packages/scheduler/src/__tests__/Scheduler-test.js
- packages/scheduler/src/__tests__/SchedulerDOM-test.js
- packages/shared/__tests__/ReactDOMFrameScheduling-test.js
- scripts/jest/setupTests.js
- scripts/rollup/bundles.js

## How did you test this change?

ci green

packages/jest-mock-scheduler/README.md

@@ -1,5 +0,0 @@
-# `jest-mock-scheduler`
-
-Jest matchers and utilities for testing the `scheduler` package.
-
-This package is experimental. APIs may change between releases.

packages/jest-mock-scheduler/npm/index.js

@@ -1,3 +0,0 @@
-'use strict';
-
-module.exports = require('scheduler/unstable_mock');

packages/jest-mock-scheduler/package.json

@@ -1,31 +0,0 @@
-{
-  "name": "jest-mock-scheduler",
-  "private": true,
-  "version": "0.1.0",
-  "description": "Jest matchers and utilities for testing the scheduler package.",
-  "main": "index.js",
-  "repository": {
-    "type" : "git",
-    "url" : "https://github.com/facebook/react.git",
-    "directory": "packages/jest-mock-scheduler"
-  },
-  "keywords": [
-    "jest",
-    "scheduler"
-  ],
-  "license": "MIT",
-  "bugs": {
-    "url": "https://github.com/facebook/react/issues"
-  },
-  "homepage": "https://reactjs.org/",
-  "peerDependencies": {
-    "jest": "^23.0.1 || ^24.0.0 || ^25.1.0",
-    "scheduler": "^0.20.0"
-  },
-  "files": [
-    "LICENSE",
-    "README.md",
-    "index.js",
-    "cjs/"
-  ]
-}