Remove workaround for bug fixed upstream

This removes a workaround that was added for a bug in
`babel-helper-compilation-targets` that was fixed upstream and is
included in the version that is required by this addon.

The bugfix in `babel-helpers-compilation-targets` was included in
[`v7.10.2`](https://github.com/babel/babel/releases/tag/v7.10.2)

`ember-cli-babel` began requiring a version of
`babel-helpers-compilation-targets` that includes the bugfix since
[`v7.21.0`](https://github.com/babel/ember-cli-babel/releases/tag/v7.21.0)

---

- [Original bugfix workaround PR](#345)
- [Dependency upgrade PR](#354)
- [`ember-cli-babel@7.21.0` Release notes](https://github.com/babel/ember-cli-babel/releases/tag/v7.21.0)
- [`babel-compilation-targets@7.10.2` Release notes](https://github.com/babel/babel/releases/tag/v7.10.2)

index.js

@@ -10,7 +10,6 @@ const {
 } = require("./lib/babel-options-util");
 
 const VersionChecker = require('ember-cli-version-checker');
-const clone = require('clone');
 const babel = require('@babel/core');
 const path = require('path');
 const getBabelOptions = require('./lib/get-babel-options');
@@ -332,14 +331,7 @@ module.exports = {
 
     let parser = require('@babel/helper-compilation-targets').default;
     if (typeof targets === 'object' && targets !== null) {
-      // babel version 7.10.0 introduced a change that mutates the input:
-      // https://github.com/babel/babel/pull/11500
-      // copy the object to guard against it, otherwise subsequent calls to
-      // _getTargets() will only have a mutated copy and lose all config from `config/targets.js`
-      // in the host application.
-      // PR to fix this upstream in babel: https://github.com/babel/babel/pull/11648
-      const copy = clone(targets);
-      return parser(copy);
+      return parser(targets);
     } else {
       return targets;
     }