This drops Read() and Slot() methods from UnalignedSlot and simplifies
Deserializer::ReadDataCase() method.

Bug: v8:8794, v8:8562
Change-Id: I8a1f62e7b36fb91e379e38d91bcbf5728d1c5471
Reviewed-on: https://chromium-review.googlesource.com/c/1456099
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59413}