Cleanup runtime/runtime-intl.cc by removing dead code, namely
Runtime_CanonicalizeLanguageTag and Runtime_GetDefaultICULocale.

Bug: v8:8439,v8:5751
Change-Id: Id8712b2f3b2ddffc0e289b904f17ae3b09faffae
Reviewed-on: https://chromium-review.googlesource.com/c/1329705
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57980}