Remove [ArrayClass]/[LegacyArrayClass] from IDLExtendedAttributes.md

[LegacyArrayClass] was removed from the WebIDL spec in
https://github.com/heycam/webidl/pull/551, so we no longer need to
include a note that it is unsupported in Chromium.

Change-Id: I32523e78b59aa74522f217b4b79cafb60d4d5eb8
Reviewed-on: https://chromium-review.googlesource.com/1036082Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Chris Nardi <cnardi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#554933}

third_party/blink/renderer/bindings/IDLExtendedAttributes.md

@@ -135,7 +135,7 @@ Extended attributes are generally not inherited: only extended attributes on the
 These are defined in the [ECMAScript-specific extended attributes](http://heycam.github.io/webidl/#es-extended-attributes) section of the [Web IDL spec](http://heycam.github.io/webidl/), and alter the binding behavior.
 
 *** note
-Unsupported: `[ArrayClass]`, `[ImplicitThis]`, `[LenientThis]`, `[NamedPropertiesObject]`, `[TreatNonCallableAsNull]`
+Unsupported: `[ImplicitThis]`, `[LenientThis]`, `[NamedPropertiesObject]`, `[TreatNonCallableAsNull]`
 ***
 
 ### [CEReactions] _(m, a)_