Remove document, frame, and iframe from fetch destinations

Per
#189 (comment),
who to associate the Save-Impression operation with is unclear in these
cases. Support for them could be restored in the future if we hear of
use cases and are able to resolve the association question.

Author: apasel422

api.bs

@@ -2021,8 +2021,7 @@ To <dfn noexport>handle Attribution headers</dfn> given a [=request=] |request|
 and [=response=] |response|, run these steps:
 
 1.  If |request|'s [=request/destination=] is not one of the following,
-    return: `""`, `"audio"`, `"document"`, `"frame"`, `"iframe"`, `"image"`,
-    `"script"`, `"track"`, `"video"`.
+    return: `""`, `"audio"`, `"image"`, `"script"`, `"track"`, `"video"`.
 
 1.  If |request|'s [=request/client=] is not a [=secure context=], return.