[css-animations] Remove note that's no longer true (or needed) now that keyframes rules cascade (as specified a few sentences following).

Thanks to Estelle Weyl for pointing this out.

Author: dbaron

css-animations/Overview.bs

@@ -245,9 +245,6 @@ Keyframes</h2>
 
 	</div>
 
-	Note: Since empty @keyframes rules are valid, they may hide the keyframes of those
-	preceding animation definitions with a matching name. 
-
 	To determine the set of keyframes, all of the values in the selectors are sorted in increasing order
 	by time. The rules within the @keyframes rule then cascade; the properties of a keyframe may thus derive
 	from more than one @keyframes rule with the same selector value.