[BUGFIX] backend_layout TCA references invalid field

The TCA definition of backend_layout defines "thumbnail=resources"
however the resources field does not exist at all in TYPO3.

The definition should be removed.

Resolves: #77353
Releases: master, 7.6
Change-Id: I76e82becfde1fbefbffc3d3b0d65b05bdc1a52cb
Reviewed-on: https://review.typo3.org/49321


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/frontend/Configuration/TCA/backend_layout.php

@@ -18,8 +18,7 @@ return array(
             'default' => 'mimetypes-x-backend_layout'
         ),
         'selicon_field' => 'icon',
-        'selicon_field_path' => 'uploads/media',
-        'thumbnail' => 'resources'
+        'selicon_field_path' => 'uploads/media'
     ),
     'interface' => array(
         'showRecordFieldList' => 'title,config,description,hidden,icon'