dawn_wire: Remove unused support of "const*const*"

This patch removes the support of annotation "const*const*" in
the template WireCmd.cpp as it is not used in current Dawn wire
implementation.

BUG=chromium:1266727

Change-Id: I4c4d68ccc050edb5f855094910440e90aef2bc4a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/72040
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>

generator/templates/api_cpp.h

@@ -140,7 +140,7 @@ namespace {{metadata.namespace}} {
     };
 
 {% macro render_cpp_default_value(member, is_struct=True) -%}
-    {%- if member.annotation in ["*", "const*", "const*const*"] and member.optional -%}
+    {%- if member.annotation in ["*", "const*"] and member.optional -%}
         {{" "}}= nullptr
     {%- elif member.type.category == "object" and member.optional and is_struct -%}
         {{" "}}= nullptr