Fix WebAudio#2150: AudioBufferOptions aren't checked again for requir…

…ed members

Just remove the bit about throwing errors if the required members are
not specified.  Let WebIDL handle this.

index.bs

@@ -2484,8 +2484,7 @@ acquired.
 
 This specifies the options to use in constructing an
 {{AudioBuffer}}. The {{AudioBufferOptions/length}} and {{AudioBufferOptions/sampleRate}} members are
-required. <span class="synchronous">A {{NotFoundError}} exception MUST be thrown if
-any of the required members are not specified.</span>
+required.
 
 <pre class="idl">
 dictionary AudioBufferOptions {