Update robots.txt to remove the * user-agent group (#13726)

Author: alex-page

polaris.shopify.com/public/robots.txt

@@ -1,8 +1,3 @@
-# Global directive for all robots
-User-agent: *
-Noai: true
-Noimageai: true
-
 # Block OpenAI's crawler
 User-agent: GPTBot
 Disallow: /
@@ -31,4 +26,4 @@ Disallow: /
 User-agent: CCBot
 Disallow: /
 User-agent: ChatGPT-User
-Disallow: /
+Disallow: /