This CL removes LoadScriptContextFieldStub and StoreScriptContextFieldStub.

Bug: v8:7206, chromium:576312
Change-Id: I217eeb726ca7d1ec85a67331da4941b9ac2a4b7a
Reviewed-on: https://chromium-review.googlesource.com/831867Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50177}