ENH: use size instead of cythonized count for fallback cases

* use size instead of cythonized count for integer case since we cannot have
  `nan` in that case
* not faster perf wise
* compilation time is shorter because count has no int templates except for
  dates and times
* any precision issues (don't think there were any) with other types are gone

Anywho, here's the vbench results for this PR vs master:

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_int_count                            |   4.0247 |   4.0337 |   0.9978 |
groupby_multi_count                          |   7.5080 |   7.4813 |   1.0036 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
```