CLN: removed setter method of categorical's ordered attribute

[gfyoung]

Deprecated back in 0.16.0 here.

[codecov-io]

Current coverage is 84.38%

Merging #13671 into master will increase coverage by <.01%

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[sinhrks]

Pls change it to check AttributeError is raised.

[gfyoung]

Done.

[gfyoung]

@sinhrks : added the test to check for AttributeError, and Travis still passing. Ready to merge if there are no other concerns.

[jreback]

thanks!

side note, can you catch these warnings (prob need to fix code to not have them emitted).

[Mon Jul 18 21:44:13 ~/pandas]$ nosetests  pandas/tests/test_categorical.py
........................................................../Users/jreback/pandas/pandas/core/internals.py:1493: FutureWarning: reshape is deprecated and will raise in a subsequent release. Please use .values.reshape(...) instead
  mask = mask.reshape(new_values.shape)
....................................................../Users/jreback/pandas/pandas/tests/test_categorical.py:4125: FutureWarning: 
Setting NaNs in `categories` is deprecated and will be removed in a future version of pandas.
  cat_exp = Categorical(["a", "b", "a"], categories=["a", "b", np.nan])
....................../Users/jreback/pandas/pandas/indexes/category.py:130: FutureWarning: 
Setting NaNs in `categories` is deprecated and will be removed in a future version of pandas.
  data = data.set_categories(categories)
.
----------------------------------------------------------------------
Ran 135 tests in 1.215s

OK

[gfyoung]

@jreback : probably something in the code would be the better fix in the future, but catching them makes sense FTTB. If I have time later, I'll give it a shot.

[jreback]

thanks!