DB/RestrictedClasses: remove unnecessary call to parent::setUpPrerequisites()

Calling `parent::setUpPrerequisites()` inside `RestrictedClassesUnitTest::enhanceGroups()` is not necessary, and it was leading to this method being called twice. PHPUnit already calls it as it used the `@before` that.

I found this in the context of the work to prepare this codebase for PHPCS 4.0 as `parent::setUpPrerequisites()` was removed from the sniff test framework in 4.0.

Originally, `RestrictedClassesUnitTest::enhanceGroups()` was called `setUp()` and called `parent::setUp()` (https://github.com/WordPress/WordPress-Coding-Standards/blob/3b9ae92607295548df357e108fd27090cf89d1d7/WordPress/Tests/DB/RestrictedClassesUnitTest.php#L34-L35). But later it was renamed and then it probably didn't made sense to continue calling `setUp()` (which was later renamed to `setUpPrerequisites()`): WordPress@00b895c.

Author: rodrigoprimo

WordPress/Tests/DB/RestrictedClassesUnitTest.php

@@ -37,8 +37,6 @@ final class RestrictedClassesUnitTest extends AbstractSniffTestCase {
 	 * @return void
 	 */
 	protected function enhanceGroups() {
-		parent::setUpPrerequisites();
-
 		AbstractFunctionRestrictionsSniff::$unittest_groups = array(
 			'test' => array(
 				'type'    => 'error',