Revert "Disallow overriding the Code-Review label."

This reverts commit 4ac6ad35a166b83fa8b934642d44b0fdc2ad5ca3.

Reason for revert: https://crrev.com/c/6872843 is landed now

Bug: 411487838
Original change's description:
> Disallow overriding the Code-Review label.
>
> Temporary, will revert after https://crrev.com/c/6872843 is landed.
>
> Bug: 411487838
> Change-Id: I57f28336a8395ae30732d90b49d7975d9fb8a614
> Reviewed-on: https://chromium-review.googlesource.com/c/infra/+/6872962
> Reviewed-by: Gregory Nisbet <gregorynisbet@google.com>

Bug: 411487838
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Id2d4d52060c85de5c557837d7db7840b9d64d19e
Reviewed-on: https://chromium-review.googlesource.com/c/infra/+/6872982
Auto-Submit: Joey Scarr <jsca@google.com>
Reviewed-by: Gregory Nisbet <gregorynisbet@google.com>