Remove outdated comment in retraction code

We're already detecting retractions that were removed from the list, so
I don't think this applies anymore.

chrome/content/zotero/xpcom/retractions.js

@@ -562,8 +562,6 @@ Zotero.Retractions = {
 		}
 		
 		if (prefixesToSend.size) {
-			// TODO: Diff list and remove existing retractions that are missing
-			
 			let possibleMatches = await this._downloadPossibleMatches([...prefixesToSend]);
 			await this._addPossibleMatches(possibleMatches, true);
 		}