Remove duplicate validation logic from webhook

[leonardoce]

This patch removes the logic to admit VolumeSnapshots, VolumeSnapshotContents, VolumeGroupSnapshots, and
VolumeGroupSnapshotContents in the validation webhook.

That logic is already implemented via CEL expressions (see #1073)

The logic to admit VolumeSnapshotClasses and VolumeGroupSnapshotClasses is still implemented in the webhook and avoids having multiple default classes for the same CSI Driver.

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:

Removes the logic that is already implemented with CEL expressions from the validation webhook.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

The validating logic for VolumeSnapshots, VolumeSnapshotContents, VolumeGroupSnapshots, and
VolumeGroupSnapshotContents has been replaced by CEL validation rules. The validating webhook
is now only being used for VolumeSnapshotClasses and VolumeGroupSnapshotClasses to ensure
that there's at most one class per CSI Driver.
The validation webhook is deprecated and will be removed in the next release. 

[xing-yang]

@leonardoce Can you please add a release note? The code change looks good to me.

[leonardoce]

Done @xing-yang. Thank you!

[xing-yang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: leonardoce, xing-yang

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