This CL also removes the JSBuiltinReducer, which is no longer needed.

Bug: v8:7340, v8:7250
Change-Id: I28896f6ce0d352047ea1cb7ea6de490818840faf
Reviewed-on: https://chromium-review.googlesource.com/1027853
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52799}