[weakrefs] Remove --no-harmony-weak-refs flag

Bug: v8:8179
Change-Id: I7f699073807d1874d0c10a4f1641de6bfb0efe6f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2741582


Commit-Queue: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73871}