DateTimeFormat's fractionalSecondDigits shipped in M84, so we
can drop the --harmony_intl_dateformat_fractional_second_digits
flag now.

Bug: v8:10485
Change-Id: If7a1a8599722073d7382635219a6fb46cdc47474
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2311410Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69007}