Clean up deep copy needed for UpdateStatefulSet

[mattcary]

/kind failing-test
/assign @liggitt
/sig apps

xref #106615

None

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[mattcary]

I wasn't able to reproduce the race locally, but I'm optimistic this is the problem.

[mattcary]

/retest

[liggitt]

/lgtm
/approve

this fix looks fine, but doesn't resolve all the flakes in the AutoDeletePVC unit test (I got a flake in the first ~25 runs running on this branch using stress):

go test -race -c ./pkg/controller/statefulset/
stress ./statefulset.test 
...
--- FAIL: TestStatefulSetControl (15.48s)
    --- FAIL: TestStatefulSetControl/CreatesPods/Monotonic/Retain/StatefulSetAutoDeletePVCEnabled (0.01s)
        stateful_set_control_test.go:196: Failed to turn up StatefulSet : claim for Pod foo-0 was not created
        stateful_set_control_test.go:204: Failed to scale statefulset to 3 replicas
        stateful_set_control_test.go:207: Failed to set ReadyReplicas correctly
        stateful_set_control_test.go:210: Failed to set UpdatedReplicas correctly

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: liggitt, mattcary

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[liggitt]

tagging into milestone to fix CI regression

[mattcary]

/lgtm /approve

this fix looks fine, but doesn't resolve all the flakes in the AutoDeletePVC unit test (I got a flake in the first ~25 runs running on this branch using stress):

go test -race -c ./pkg/controller/statefulset/
stress ./statefulset.test 
...
--- FAIL: TestStatefulSetControl (15.48s)
    --- FAIL: TestStatefulSetControl/CreatesPods/Monotonic/Retain/StatefulSetAutoDeletePVCEnabled (0.01s)
        stateful_set_control_test.go:196: Failed to turn up StatefulSet : claim for Pod foo-0 was not created
        stateful_set_control_test.go:204: Failed to scale statefulset to 3 replicas
        stateful_set_control_test.go:207: Failed to set ReadyReplicas correctly
        stateful_set_control_test.go:210: Failed to set UpdatedReplicas correctly

Cool, I didn't know about stress!

These look like different flakes--not a mutation detector race---so probably better to fix in another PR anyway.