Retire IntersectionObserverV2 runtime flag

The flag IntersectionObserverV2 has been stable since 2019-01.

Bug: 827639
Change-Id: I31e4e6b5a0cdae99a0a3c56d504a99216cc653b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2208802


Reviewed-by: Stefan Zager <szager@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772052}