As builtins no longer uses default_microtask_queue after
https://crrev.com/c/1369906, Isolate's default_microtask_queue_address()
is no longer needed.

Bug: v8:8124
Change-Id: I1f51e58ada1a62f71f19f6c15b2330dfed176839
Reviewed-on: https://chromium-review.googlesource.com/c/1420178Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58930}