Remove obsolete code

Code was identical with the default implementation in `AbstractAttachmentObjectType`

Author: BurntimeX

wcfsetup/install/files/lib/system/attachment/ContactAttachmentObjectType.class.php

@@ -64,14 +64,6 @@ public function canDownload($objectID)
         return true;
     }
 
-    /**
-     * @inheritDoc
-     */
-    public function canViewPreview($objectID)
-    {
-        return $this->canDownload($objectID);
-    }
-
     /**
      * @inheritDoc
      */

wcfsetup/install/files/lib/system/attachment/SignatureAttachmentObjectType.class.php

@@ -47,14 +47,6 @@ public function canDownload($objectID)
         return $this->canAddUser();
     }
 
-    /**
-     * @inheritDoc
-     */
-    public function canViewPreview($objectID)
-    {
-        return $this->canDownload($objectID);
-    }
-
     /**
      * @inheritDoc
      */