REF: remove _transform_item_by_item (pandas-dev#51224)

Author: jbrockmendel

pandas/core/groupby/generic.py

@@ -1542,9 +1542,8 @@ def _transform_general(self, func, *args, **kwargs):
             object.__setattr__(group, "name", name)
             try:
                 path, res = self._choose_path(fast_path, slow_path, group)
-            except TypeError:
-                return self._transform_item_by_item(obj, fast_path)
             except ValueError as err:
+                # e.g. test_transform_with_non_scalar_group
                 msg = "transform must return a scalar value for each group"
                 raise ValueError(msg) from err
             if group.size > 0:
@@ -1677,17 +1676,6 @@ def _choose_path(self, fast_path: Callable, slow_path: Callable, group: DataFram
 
         return path, res
 
-    def _transform_item_by_item(self, obj: DataFrame, wrapper) -> DataFrame:
-        # iterate through columns, see test_transform_exclude_nuisance
-        #  gets here with non-unique columns
-        output = {}
-        for i, (colname, sgb) in enumerate(self._iterate_column_groupbys(obj)):
-            output[i] = sgb.transform(wrapper)
-
-        result = self.obj._constructor(output, index=obj.index)
-        result.columns = obj.columns
-        return result
-
     def filter(self, func, dropna: bool = True, *args, **kwargs):
         """
         Filter elements from groups that don't satisfy a criterion.