[Activities] - Remove no-longer-valid preview callout for url mappings (discord#7218)

Author: matthova

docs/activities/Development_Guides.mdx

@@ -209,9 +209,6 @@ Because your application is "sandboxed", it will be unable to make network reque
 
 #### How to set a URL Mapping
 
-> preview
-> This is only possible once Activities are enabled on your application.
-
 To add or modify your application's URL mappings, click on `Activities -> URL Mappings` and set the prefix and target values for each mapping as needed.
 
 ![Configuring your URL Mapping](activities/url-mapping-tutorial.png)