Drop obsolete comment

Author: oliverklee

src/CSSList/KeyFrame.php

@@ -7,10 +7,6 @@
 use Sabberworm\CSS\OutputFormat;
 use Sabberworm\CSS\Property\AtRule;
 
-/**
- * Note: When constructing an instance of this class, `setVendorKeyFrame()` must be called directly after construction
- * for the instance to be valid.
- */
 class KeyFrame extends CSSList implements AtRule
 {
     /**