Cleans up always=true intl-relative-time-format flag
It shipped in m71 in Dec 2018.

Bug: v8:8704
Change-Id: I52d86aea9aedf201a216a1df0773a486fbee37b9
Reviewed-on: https://chromium-review.googlesource.com/c/1417299Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58904}