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Use single struct in internal type to reduce difference with external type

**What this PR does / why we need it**:
This PR reduces the differences between internal and external types by removing leftovers after revert of one of the changes.

**Special notes for your reviewer**:
We had `IDRange` in both types prior 9440a687 commit (https://github.com/kubernetes/kubernetes/pull/44714) that splitted it into `UserIDRange`/`GroupIDRange`. Later, in c91a12d2 commit (https://github.com/kubernetes/kubernetes/pull/47824) we had to revert these changes because they broke backward compatibility but `UserIDRange`/`GroupIDRange` structs were left in the internal type.