Revert of [api] Fix compilation error for `UNIMPLEMENTED()` method (patchset #1 id:1 of https://codereview.chromium.org/2929993003/ )

Reason for revert:
not needed any more, and contradicts our cleanup efforts: https://chromium-review.googlesource.com/c/507287/

Original issue's description:
> [api] Fix compilation error for `UNIMPLEMENTED()` method
>
> Return `nullptr` from `ArrayBuffer::Allocator::Reserve` because
> apparently not doing so results in compile errors for some people.
>
> BUG=
>
> Ref: https://github.com/nodejs/node/issues/13392
> Review-Url: https://codereview.chromium.org/2929993003
> Cr-Commit-Position: refs/heads/master@{#45886}
> Committed: https://chromium.googlesource.com/v8/v8/+/f14d1b62313329f421f12cae429673c1b13018f9

R=franzih@chromium.org,addaleax@gmail.com

Review-Url: https://codereview.chromium.org/2946933002
Cr-Commit-Position: refs/heads/master@{#46739}