BUG: Float64Index.get_value() for tuples.
- [x] closes #13509
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Currently, trying to retrieve one tuple-valued entry in a `Series` with
a `Float64Index` will fail (GH 13509). This fixes that bug.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14092?src=pr) is 85.26% (diff: 100%)

> Merging [#14092](https://codecov.io/gh/pydata/pandas/pull/14092?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14092   diff @@
==========================================
  Files           139        139          
  Lines         50492      50486     -6   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43053      43049     -4   
+ Misses         7439       7437     -2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [185fcbe...2b77554](https://codecov.io/gh/pydata/pandas/compare/185fcbebd8c65706a9e3fea2d1ae28aa2d8baa93...2b7755419c7e5140cbe70f935ad78d87f3d0b6c1?src=pr)
 thanks for the fix!

that was completely dead code that you removed! (except of course _your_ example hit it!)
