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
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13676?src=pr) is **84.38%**

> Merging [#13676](https://codecov.io/gh/pydata/pandas/pull/13676?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13676   diff @@
==========================================
  Files           142        142          
  Lines         51223      51223          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43223      43224     +1   
+ Misses         8000       7999     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [d7c028d...bdfd2cc](https://codecov.io/gh/pydata/pandas/compare/d7c028d4965932160fa3b69f56c716b1454c42a5...bdfd2cc3f390e3d407203548d5eaea453f86075a?src=pr)
 yes this was annoying!
