BUG: removes warning when a column name may collide with a method

Closes pandas-dev#17268 and reverts work from pandas-dev#16951 which warns users when creating
a column whose name matches one of the methods defined on ndframes (e.g.
'sum' or 'mean'). The warning was deemed to be too much of a hindrance to
the common use case of generating dataframes of summary statistics.

Author: deniederhut

pandas/core/generic.py

@@ -1905,10 +1905,6 @@ def _slice(self, slobj, axis=0, kind=None):
         return result
 
     def _set_item(self, key, value):
-        if isinstance(key, str) and callable(getattr(self, key, None)):
-            warnings.warn("Column name '{key}' collides with a built-in "
-                          "method, which will cause unexpected attribute "
-                          "behavior".format(key=key), stacklevel=3)
         self._data.set(key, value)
         self._clear_item_cache()