Now that no harmony-flagged features are implemented in experimental
JS, most of this is simply dead code.

As PostExperimentals() is no longer needed, I also removed the use of
Import() in the debug context, allowing the deletion of PostDebug()
along with PostExperimentals(); cleanup code is moved to the
end of PostNatives.

Also gets rid of some longer-dead code in prologue.js related to
TypedArrays, and some duplicate code for setting up SharedArrayBuffer
builtins.

Review-Url: https://codereview.chromium.org/2705293004
Cr-Commit-Position: refs/heads/master@{#43418}