flags: Remove flag from finished experiment

This CL removes the await-optimization-flag, that is no longer needed,
since the feature is considered web compatible.

Intent to ship discussions:
https://groups.google.com/forum/#!topic/v8-users/ML7gOgK8Dsg
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/74QxqINq4g0

Bug: v8:8267, chromium:893469
Change-Id: I308d38811bfac3a55392a1e07e99ce702f57a7d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617517
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662105}
8 files changed