TST: assert message shows unnecessary diff
- [x] passes `git diff upstream/master | flake8 --diff`

removed unnecessary trailing `None`. 

```
pd.util.testing.assert_index_equal(pd.Index([1, 2, 3]), pd.Index([1 ,2, 4]))
# AssertionError: Index are different
# 
# Index values are different (33.33333 %)
# [left]:  Int64Index([1, 2, 3], dtype='int64')
# [right]: Int64Index([1, 2, 4], dtype='int64')None
```
