Correct the syntax for registring global namespace. (#6323)

* Correct the syntax for registring global namespace.

* Remove unused ViewHelpers import

Removed unused import of MyVendor\MyExtension\ViewHelpers.

* Add namespace configuration for myextension

Author: Oktopuce

Documentation/ApiOverview/Fluid/_Syntax/_ext_localconf.php

@@ -2,10 +2,8 @@
 
 declare(strict_types=1);
 
-use MyVendor\MyExtension\ViewHelpers;
-
 defined('TYPO3') or die();
 
 $GLOBALS['TYPO3_CONF_VARS']['SYS']['fluid']['namespaces']['myextension'] = [
-    ViewHelpers::class,
+    'MyVendor\\MyExtension\\ViewHelpers',
 ];