docs: drop the last references to cover scraping (#60)

Review follow-ups. The README's adapter reference and the userscript
testing skill still described a scrape that no longer exists, and the
manga test stub kept a querySelectorAll whose only caller was the
deleted coverFromPage. The handler comment's premise ("every installed
userscript still sends one") stops being true the moment a Reader
reinstalls, so it now says older copies may.

userscript/test/logic.test.js

@@ -41,9 +41,6 @@ globalThis.document = {
     const v = metaTags[m[1]];
     return v == null ? null : { getAttribute: () => v };
   },
-  querySelectorAll() {
-    return [];
-  },
   addEventListener() {},
   get title() {
     return docTitle;