Remove the gate "SkipReadOnlyValidationGCE"

[thockin]

One less GCE-specifc wart in the codebase.

xref #114436
xref #121083
xref #117531

/kind cleanup

The feature gate "SkipReadOnlyValidationGCE" has been removed.  This gate has been active for 2 releases with no reports of issues (and was such a niche thing, we didn't expect any).

[k8s-ci-robot]

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[k8s-ci-robot]

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[thockin]

/test pull-kubernetes-e2e-gce

[pacoxu]

/lgtm
/sig apps

cc @mfordjody

BTW, kubernetes/website#44596 is updating the FG description for current master(1.30). We will remove the part in v1.31 dev later.

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 74e1f24bbf9b23b63c9223d6bdef8c0f3934f738