UPSTREAM: 64971: Cherry pick of #64255: fix field removal in mutating admission webhooks

[pmorie]

Needed to make knative run smoothly on 3.10

[openshift-ci-robot]

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[pmorie]

/retest

[openshift-ci-robot]

@pmorie: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
ci/openshift-jenkins/end_to_end	6efefed	link	/test end_to_end

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[smarterclayton]

This cherrypick needs to be release-3.10 to go into origin.

[pmorie]

Wrong branch

[pmorie]

/retest