Cleaned up return of _get_cython_function (pandas-dev#19561)

Author: WillAyd

pandas/core/groupby.py

@@ -2240,7 +2240,7 @@ def wrapper(*args, **kwargs):
             raise NotImplementedError("function is not implemented for this"
                                       "dtype: [how->%s,dtype->%s]" %
                                       (how, dtype_str))
-        return func, dtype_str
+        return func
 
     def _cython_operation(self, kind, values, how, axis, min_count=-1):
         assert kind in ['transform', 'aggregate']
@@ -2304,12 +2304,12 @@ def _cython_operation(self, kind, values, how, axis, min_count=-1):
             values = values.astype(object)
 
         try:
-            func, dtype_str = self._get_cython_function(
+            func = self._get_cython_function(
                 kind, how, values, is_numeric)
         except NotImplementedError:
             if is_numeric:
                 values = _ensure_float64(values)
-                func, dtype_str = self._get_cython_function(
+                func = self._get_cython_function(
                     kind, how, values, is_numeric)
             else:
                 raise