fix some audience parsing assumptions

The assumptions made in the comment is actually wrong. The comment says:
"If value is a string, this means it must have passed through the resolver,
which means it must be public." But this is not true because we do signed
GET requests which means we may well get non-public posts using the
resolver.

packages/backend/src/remote/activitypub/models/note.ts

@@ -101,10 +101,8 @@ export async function createNote(value: string | IObject, resolver: Resolver, si
 
 	// If audience(to,cc) was not specified
 	if (visibility === 'specified' && visibleUsers.length === 0) {
-		if (typeof value === 'string') {	// 入力がstringならばresolverでGETが発生している
-			// こちらから匿名GET出来たものならばpublic
-			visibility = 'public';
-		}
+		// TODO derive audience from context (e.g. whose inbox this was in?)
+		throw new Error('audience not understood');
 	}
 
 	let isTalk = note._misskey_talk && visibility === 'specified';