refactor #128 Remove performNoDeepMerging to authorise additional cus…

…tom theme folders (lanfisis)

This PR was merged into the 2.4 branch.

Discussion
----------

The `performNoDeepMerging` do not allow to add more theme folders in config file or using compiler pass so I removed it. 

Commits
-------

e17a77f Remove performNoDeepMerging to authorise theme folder addition

src/Configuration/Filesystem/FilesystemConfigurationSourceFactory.php

@@ -45,7 +45,6 @@ public function buildConfiguration(ArrayNodeDefinition $node): void
             ->arrayNode('directories')
                 ->defaultValue(['%kernel.project_dir%/themes'])
                 ->requiresAtLeastOneElement()
-                ->performNoDeepMerging()
                 ->prototype('scalar')
         ;
     }