It's not clear what this logic is there for; ICU seems to already
preserve the locale in the way that the comment mentions. There
appear to be tests in test/intl/general/mapped-locale.js which
remain passing.

Bug: v8:5751
Change-Id: Ib9c64f00b982711ae0eab078252a88f44b81b894
Reviewed-on: https://chromium-review.googlesource.com/485780
Commit-Queue: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45080}