This reverts commit 07c1e641.

Reason for revert: Breaks TSAN build.

https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20TSAN/builds/19784

Original change's description:
> [in-place weak refs] Add in-place weak references & migrate one WeakCell to it.
> 
> Implement in-place weak reference handling in GC.
> 
> Turn FeedbackVector::optimized_code_or_smi into an in-place weak reference (this
> is the only in-place weak reference at this point).
> 
> (See bug for design doc.)
> 
> BUG=v8:7308
> 
> Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng;master.tryserver.chromium.linux:linux_chromium_rel_ng
> Change-Id: I0f9f992cb4ee0457c40b7c868317dfb607bfb906
> Reviewed-on: https://chromium-review.googlesource.com/873638
> Commit-Queue: Marja Hölttä <marja@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Reviewed-by: Hannes Payer <hpayer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51722}

TBR=ulan@chromium.org,marja@chromium.org,yangguo@chromium.org,hpayer@chromium.org,mlippautz@chromium.org,ishell@chromium.org,bmeurer@chromium.org

Change-Id: I75a7dd99fbfd2f5922a6c4d2000bea2adfdeac11
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7308
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng;master.tryserver.chromium.linux:linux_chromium_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/948522Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51723}