Drive-by cleanup: remove megamorphic stub cache lookup support from generic property
store code. This lookup is no longer necessary because
1) fast stores to existing properties get all the information from the map,
2) transitioning store targets are taken directly from the transition array,
so in both cases there's no point in doing a store handler lookup.

Bug: v8:5988
Change-Id: I95c0a08e7d1a76bb0f4475a9bd685e4e11e16a48
Reviewed-on: https://chromium-review.googlesource.com/983921
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52349}