Remove a template param of LinkedHashSet in stub

This is a follow up for https://crrev.com/c/2134039 which removes a
HashArg param of LinkedHashSet in stub.

Bug: 1100257
Change-Id: Ib43fd54fa553b7228f97c128598e385020feeaf9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377533Reviewed-by: Keishi Hattori <keishi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Yuki Yamada <yukiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802490}

tools/clang/blink_gc_plugin/tests/heap/stubs.h

@@ -102,7 +102,6 @@ class ListHashSet {
 };
 
 template <typename ValueArg,
-          typename HashArg = void,
           typename TraitsArg = void,
           typename Allocator = DefaultAllocator>
 class LinkedHashSet {