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

Bug: v8:8267, chromium:893469
Change-Id: I046f003f9ed6853d713b825c436fe5f0a1e5a0a9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1630677Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61834}