[cleanup] Remove obsolete forward declaration.

R=marja@chromium.org

Bug: 
Change-Id: Ic7a2fc28baa5ecf0650287fbc193ff0eccbf2f5e
Reviewed-on: https://chromium-review.googlesource.com/612248
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47312}

src/ast/ast.h

@@ -116,7 +116,6 @@ namespace internal {
 // Forward declarations
 class AstNodeFactory;
 class Declaration;
-class Module;
 class BreakableStatement;
 class Expression;
 class IterationStatement;