ENH: No need to call _choose_path in Dataframe groupby transform function 
When every time the slow_path method run in self._choose_path, then why not we remove the fast_path code.

self._choose_path call
https://github.com/pandas-dev/pandas/blob/d7a5b838d8d6234f6bec5a30bfa33b24bd4afbd9/pandas/core/groupby/generic.py#L1313

Line to change
path, res = show_path, slow_path(group)


definition of self._choose_path method
https://github.com/pandas-dev/pandas/blob/d7a5b838d8d6234f6bec5a30bfa33b24bd4afbd9/pandas/core/groupby/generic.py#L1425

Execution time difference
----------------------------------
With fast_path method - 21 ms
Without fast_path method - 9ms

Used dataframe
df = pd.DataFrame({'A': ['foo', 'bar', 'foo', 'bar', 'foo', 'bar'], 'B': ['one', 'one', 'two', 'three', 'two', 'two'],  'C': [10, 5, 50, 2, 50, 5],  'D': [2.0, 5., 8., 1., 2., 9.]})
