Before, we carefully turned on fast array builtins only if flag
--enable-fast-array-builtins was true (though it was implied true
if --turbo was on). Now, the set of Array.prototype.{some, forEach,
every, reduce} is good enough to always turn them on. This means
we can remove the JavaScript implementations.

The flag is renamed to --experimental-fast-array-builtins, which is
off. In the next days we'll add more non-javascript implementations
here for testing.

BUG=
R=danno@chromium.org

Review-Url: https://codereview.chromium.org/2761783002
Cr-Commit-Position: refs/heads/master@{#44026}