It seems the trashcan option when viewing plan details is available beginning with Data Studio 2.5. In earlier releases, you will need to use the Undo option to remove the last transform added. In your case, the transform is last, so you should only need to click Undo once. If the transform that you want to delete is in the middle or beginning, you will need to click Undo for each transform until you get to the one you want to delete. Then you will need to add back the transforms you want to keep.
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