CLN: simplify get_values usage in groupby
This will avoid an object-casting for e.g. IntervalArray-backed ExtensionBlock.
Or if overkill just a quick timeit  > Or if overkill just a quick timeit

```
ts = pd.Timestamp("2016-01-01", tz="UTC")
ii = pd.interval_range(start=ts, end=ts + pd.Timedelta(days=10**4))       
df = pd.DataFrame({"A": ii.right, "B": ii.left, "C": ii, "D": ii.left.year})
gb = df.groupby("D")

In [5]: %timeit gb.count()                                                                                                                                                                            
378 µs ± 6.23 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)  # <-- PR
211 ms ± 18.4 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)  # <-- master
```