CLN: Removed levels attribute from Categorical
Deprecated back in `0.15.0` and therefore long overdue.  Closes #8376.

Looks good to me

cc @janschulz 
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13612?src=pr) is **84.38%**

> Merging [#13612](https://codecov.io/gh/pydata/pandas/pull/13612?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13612   diff @@
==========================================
  Files           142        142          
  Lines         51235      51223    -12   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43235      43226     -9   
+ Misses         8000       7997     -3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [c9a27ed...f1254df](https://codecov.io/gh/pydata/pandas/compare/c9a27ede0925ddbaa8d3ec9efd3c332a636505cf...f1254dfc03bee29120f844af2f0ff515fbe8ae82?src=pr)
 @jreback , @janschulz : relocated those pickle tests for `Categorical`, and Travis is still happy.  Ready to merge if there are no other concerns.
 thanks!
