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

Calling `AbstractSniffUnitTest::setUpPrerequisites()` manually inside `RestrictedClassesUnitTest::enhanceGroups()` is not necessary as this method already executes via its `@before` tag and results in the method running twice.

Originally, `RestrictedClassesUnitTest::enhanceGroups()` was named `RestrictedClassesUnitTest::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 make sense to continue calling `parent::setUp()` (which was later renamed to `setUpPrerequisites()`): 00b895c.

I found this in the context of the work to prepare this codebase for PHPCS 4.0 as `AbstractSniffUnitTest::setUpPrerequisites()` was renamed in 4.0 (PHPCSStandards/PHP_CodeSniffer@92cf10f#diff-91ba2a2a923bbfdaa61dcd31051d783bd0e7ff99f979f6e1a2917cae7dde3eb2L58).

Author: rodrigoprimo

WordPress/Tests/DB/RestrictedClassesUnitTest.php

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