Remove async as a keyword

Fixes #1508.

Author: saschanaz

index.bs

@@ -469,7 +469,6 @@ underscore.
 
 <pre class="grammar" id="prod-ArgumentNameKeyword">
     ArgumentNameKeyword :
-        "async"
         "attribute"
         "callback"
         "const"
@@ -1807,7 +1806,6 @@ are applicable only to regular attributes:
 
 <pre class="grammar" id="prod-AttributeNameKeyword">
     AttributeNameKeyword :
-        "async"
         "required"
 </pre>