Merged WasmCode::Tier into Execution Tier.

Bug: v8:9003
Change-Id: I0ad439b8bc060f73e71d60ab9c93dd6bc18d05fe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1547852
Commit-Queue: Frederik Gossen <frgossen@google.com>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60610}