As per
https://unicode-org.atlassian.net/browse/ICU-20009?focusedCommentId=62380&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-62380
we can just use DateFormat::GetAvailableLocales for RelativeTimeFormat
removing a lot of code.

This patch also cleans up the code to be in line with the rest of V8.

Bug: v8:5751
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I8df5b0e5a9a05c426aaa4f7fb9c67d7947301478
Reviewed-on: https://chromium-review.googlesource.com/c/1237298
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56455}