Groupby count doesn't accept sort= keyword
Many groupby aggregations like sum, min, max, mean, and so on take a `sort=` keyword.  Count doesn't for some reason.

```python
In [1]: import pandas as pd

In [2]: df = pd.DataFrame({"x": [3, 4, 1, 3, 4, 1], "y": [1, 2, 3, 4, 5, 6]})

In [3]: df.groupby("x").y.sum(sort=False)
Out[3]:
x
1    9
3    5
4    7
Name: y, dtype: int64

In [4]: df.groupby("x").y.count(sort=False)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-76e5507e57cb> in <module>
----> 1 df.groupby("x").y.count(sort=False)

TypeError: count() got an unexpected keyword argument 'sort'
```

Is this intentional?  Is it a bug?  I was trying to turn off sorting intermediate results in dask dataframe by default to speed things up a bit (it's also for cudf/RAPIDS work) and ran into this.  The inconsistency makes things slightly inconvenient.