Change robots.txt to exclude only media proxy URLs (mastodon#10038)

* Revert "Change robots.txt to exclude some URLs (mastodon#10037)"

This reverts commit 80161f4.

* Let's block media_proxy

/media_proxy/ is a dynamic route used for requesting uncached media, so it's
probably bad to let crawlers use it

* misleading comment

public/robots.txt

@@ -1,13 +1,4 @@
-User-Agent: *
-Disallow: /users/*/followers
-Disallow: /users/*/following
-Disallow: /@*/media
-Disallow: /@*/with_replies
-Disallow: /@*/tagged/*
-Disallow: /media_proxy/*
-Disallow: /emoji/*
-Disallow: /packs/*
-Disallow: /sounds/*
-Disallow: /system/*
-Disallow: /avatars/*
-Disallow: /headers/*
+# See http://www.robotstxt.org/robotstxt.html for documentation on how to use the robots.txt file
+
+User-agent: *
+Disallow: /media_proxy/