Remove MinDomainsInPodTopologySpread feature gate

[sanposhiho]

ref: kubernetes/enhancements#3022

This feature is already GA, but we haven't removed the feature gate yet on purpose.
We can remove this feature gate after a few releases, probably 1.32 -ish.

Yes, we do keep the feature gate for a couple of releases, so that someone that a manifest that has MyFeature=true continues to run. But we ignore the feature gate in code.
It is common that we forget to remove the gate.
kubernetes/enhancements#4407 (comment)

/kind cleanup
/assign
/sig scheduling

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