[PERF] Get rid of MultiIndex conversion in IntervalIndex.is_unique 
-  xref #24813 
-  benchmark added
-  passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
-  whatsnew entry

This PR follows #25159 (in which I broke something when merging).
```
All benchmarks:

       before           after         ratio
     [304d8d4b]       [4ec1fe97]
     <master>         <intv-is-unique>
        228±0.2ns        230±0.2ns     1.01  index_object.IntervalIndexMethod.time_is_unique(1000)
-         450±7ns          277±2ns     0.62  index_object.IntervalIndexMethod.time_is_unique(100000)
```

The approach this time doesn't worsen the performance.