[web-animations-2] Fix WebIDL - no inherit for partial dictionary

Per WebIDL, a partial dictionary declaration cannot inherit from a base
dictionary. See Note in:
https://heycam.github.io/webidl/#idl-interfaces

"Note: A partial interface definition cannot specify that the interface inherits
from another interface. Inheritance must be specified on the original interface
definition."

`ComputedEffectTiming`, defined in Level 1, already inherits from
`EffectTiming`, so no need to formulate that again in the partial definition.

Author: tidoust

web-animations-2/Overview.bs

@@ -2085,7 +2085,7 @@ partial dictionary OptionalEffectTiming {
   <code>ComputedEffectTiming</code> dictionary</h3>
 
 <pre class='idl'>
-partial dictionary ComputedEffectTiming : EffectTiming {
+partial dictionary ComputedEffectTiming {
     double startTime;
 };
 </pre>