DEV: Remove Discourse.redis.delete_prefixed (#22103)

This method is a huge footgun in production, since it calls
the Redis KEYS command. From the Redis documentation at
https://redis.io/commands/keys/:

> Warning: consider KEYS as a command that should only be used in
production environments with extreme care. It may ruin performance when
it is executed against large databases. This command is intended for
debugging and special operations, such as changing your keyspace layout.
Don't use KEYS in your regular application code.

Since we were only using `delete_prefixed` in specs (now that we
removed the usage in production in 24ec06ff85c7acbad9621092b5e50eec2ede7b83)
we can remove this and instead rely on `use_redis_snapshotting` on the
particular tests that need this kind of clearing functionality.

lib/discourse_redis.rb

@@ -213,10 +213,6 @@ class DiscourseRedis
     end
   end
 
-  def delete_prefixed(prefix)
-    DiscourseRedis.ignore_readonly { keys("#{prefix}*").each { |k| Discourse.redis.del(k) } }
-  end
-
   def reconnect
     @redis._client.reconnect
   end