release-23.2: gcs: remove test that asserts buckets produce not found error

[blathers-crl]

Backport 1/1 commits from #135565 on behalf of @jeffswenson.

/cc @cockroachdb/release

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The behavior of GCS appears to have changed. If a bucket does not exist,
it will return a 403 forbidden. So we can't disambiguate a bucket that
does not exist from a bucket we can't access.

Overall, this is annoying but fine. The main value of not found as a
sentinel error the program can check is it indicates the name is free
and can be used. This property doesn't apply if the bucket does not
exist because CRDB will never create a bucket.

Release Justification: Test only change
Release Note: None
Fixes: #135307
Fixes: #135348
Fixes: #135349
Fixes: #135350
Fixes: #135351
Fixes: #135352

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Release justification:

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[msbutler]

while i'm fine deleting this test (i dont see it adding much value), we should probably remove the comment added to external_storage.go