Simplified test for default locale under test/intl
Remove GetDefaultLocale from runtime
Move Intl::DefaultLocale from intl-object.h to
internal function inside intl-object.cc

Bug: v8:5751
Change-Id: I885abf30ff33d5213ee99c07ac1e92d3c5065d8b
Reviewed-on: https://chromium-review.googlesource.com/c/1358022Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Jungshik Shin <jshin@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58081}