CLN: removed the 'diff' method for Index
Deprecated all the way back in `0.15.0` <a href="https://github.com/pydata/pandas/pull/8227">here</a>.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13669?src=pr) is **84.52%**

> Merging [#13669](https://codecov.io/gh/pydata/pandas/pull/13669?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13669   diff @@
==========================================
  Files           141        141          
  Lines         51145      51144     -1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43233      43232     -1   
  Misses         7912       7912          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [8acfad3...7dca659](https://codecov.io/gh/pydata/pandas/compare/8acfad343c88760a6d09fea221996dd50393fa8a...7dca65980f0245e4602e42393bdb9e715a6efe93?src=pr)
 ty!
