CLN: Removed SparsePanel
Title is self-explanatory.  Picks up where #11157 left off.

this is in slight conflict with #13776 as I needed to add a line to catch an errant warning about SparsePanel (which you should then remove). But can sort that later I guess.
 @jreback : what error catching are you referring to in your PR?
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13778?src=pr) is 85.23% (diff: 75.00%)

> Merging [#13778](https://codecov.io/gh/pydata/pandas/pull/13778?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **0.10%**

``` diff
@@             master     #13778   diff @@
==========================================
  Files           141        140     -1   
  Lines         50711      50415   -296   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43275      42969   -306   
- Misses         7436       7446    +10   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [3fdcea6...f3fa93b](https://codecov.io/gh/pydata/pandas/compare/3fdcea6f89a5a9f9fdb98452343a21e576430bd9...f3fa93bb9701925916352a52f865d17be0f38f86?src=pr)
 lgtm @gfyoung minor doc changes. ping when green.
 @gfyoung I think I took out the reference in my other PR (to SparsePanel); I'll rebase after this.
 thanks @gfyoung 
