FIX: Drop remaining reference to Atlas

src/derivatives/imaging.md

@@ -155,8 +155,7 @@ A binary (1 - inside, 0 - outside) mask in the space defined by the [`space` ent
 If no transformation has taken place, the value of `space` SHOULD be set to `orig`.
 If the mask is an ROI mask derived from an atlas segmentation,
 then the [`label` entity](../appendices/entities.md#label)) SHOULD be used to specify the masked structure
-(see [Common image-derived labels](#common-image-derived-labels)),
-and the `Atlas` metadata SHOULD be defined.
+(see [Common image-derived labels](#common-image-derived-labels)).
 
 JSON metadata fields: