Keep only the bot cookie for the last visited bot

A workspace conversation is never remembered, so /admin/bots always
reopens the bot's own chat.

src/utils/read-last-visited-bot.ts

@@ -7,14 +7,13 @@ type Params = {
 };
 
 /**
- * The conversation this browser was last in, from the last-visited cookies
- * (see `lastVisitedBotCookies`). Null when it has none, or when they don't
- * name a conversation. Whether the user still reaches it is the caller's
+ * The bot this browser was last in, from the last-visited cookie (see
+ * `lastVisitedBotCookies`). Null when it has none, or when it doesn't name
+ * a bot. Whether the user still reaches that bot's own chat is the caller's
  * check: a stale grant must fall back, never redirect.
  */
 export default function readLastVisitedBot({ req }: Params): LastVisitedBot | null {
     return parseLastVisitedBot({
         bot_id: getCookie(req, AppData["LastVisitedBotCookieName"]),
-        workspace_id: getCookie(req, AppData["LastVisitedWorkspaceCookieName"]),
     });
 }