The previous %StringCharCodeAt runtime entry (and the inlined intrinsic)
are obsolete and not used anymore (except in dedicated tests for this
runtime function), so remove it. And rename the %StringCharCodeAtRT
function, which is actually used to %StringCharCodeAt instead to have
a consistent naming scheme for runtime fallbacks.

Bug: v8:5049
Change-Id: I619429ef54f6efea61fc51ab9ed1d5cfe4417f99
Reviewed-on: https://chromium-review.googlesource.com/657719
Commit-Queue: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47928}