docs: remove GM_addStyle(css) doc

Author: quoid

README.md

@@ -177,10 +177,6 @@ Userscripts currently supports the following api methods. All methods are asynch
 - `GM.addStyle(css)`
     - `css: String`
     - on success returns a promise resolved with the css string argument provided
-- `GM_addStyle(css)`
-    - `css: String`
-    - **synchronous**
-    - returns the css string argument provided *without regard to success*
 - `GM.setValue(key, value)`
     - `key: String`, `value: Any`
     - on success returns a promise resolved with an object indicating success