Revert "Dark autoloader magic for ThemingDefaults"

This reverts commit 4e1d501.

lib/private/Server.php

@@ -1199,16 +1199,10 @@ public function __construct($webRoot, \OC\Config $config) {
 
 		$this->registerAlias(\OC_Defaults::class, 'ThemingDefaults');
 		$this->registerService('ThemingDefaults', function (Server $c) {
-			/*
-			 * Dark magic for autoloader.
-			 * If we do a class_exists it will try to load the class which will
-			 * make composer cache the result. Resulting in errors when enabling
-			 * the theming app.
-			 */
-			$prefixes = \OC::$composerAutoloader->getPrefixesPsr4();
-			if (isset($prefixes['OCA\\Theming\\'])) {
-				$classExists = true;
-			} else {
+			try {
+				$classExists = class_exists('OCA\Theming\ThemingDefaults');
+			} catch (\OCP\AutoloadNotAllowedException $e) {
+				// App disabled or in maintenance mode
 				$classExists = false;
 			}