[web-animations-2] Remove inheritance from partial interface definition

Inheritance is already set in the main interface declaration, and is syntactically forbidden at the partial level.

Author: dontcallmedom

web-animations-2/Overview.bs

@@ -2076,7 +2076,7 @@ Update the startTime and currentTime of the Animation interface as follows:
 
 <pre class="idl">
 [Exposed=Window]
-partial interface Animation : EventTarget {
+partial interface Animation {
     attribute CSSNumberish?       startTime;
     attribute CSSNumberish?       currentTime;
 };