This reverts commit d7b67ce2.

Reason for revert: broke tsan https://logs.chromium.org/logs/v8/buildbucket/cr-buildbucket.appspot.com/8901789268326050304/+/steps/Check/0/logs/enumeration-order/0

Original change's description:
> [Context] Add a bit flag to indicate if extension might exist
> 
> Checking the bit flag instead of comparing pointers should improve performance.
> This will also allow us to remove the extension slot in Context and save memory.
> 
> Bug: v8:9744
> Change-Id: I7ab9feeadfb934955798d877d13bc0e1d78a191c
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1814918
> Commit-Queue: Victor Gomes <victorgomes@google.com>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#63906}

TBR=ulan@chromium.org,leszeks@chromium.org,victorgomes@google.com

Change-Id: I3d2261e24c9c7da5f5a1d49803361bc6f0770330
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:9744
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1816514Reviewed-by: Sathya Gunasekaran  <gsathya@chromium.org>
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63907}