CLN: Remove never-True Block.is_sparse
Along with some non-reachable branches in Block.quantile.  This in turn lets us get rid of Block._na_value.
Hello @jbrockmendel! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:









There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2019-06-25 14:21:32 UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27037?src=pr&el=h1) Report
> Merging [#27037](https://codecov.io/gh/pandas-dev/pandas/pull/27037?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f0919f272d9614058b5ebb5e0664d1ac6f23540f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/27037/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/27037?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #27037      +/-   ##
==========================================
+ Coverage   91.99%      92%   +<.01%     
==========================================
  Files         180      180              
  Lines       50774    50757      -17     
==========================================
- Hits        46712    46698      -14     
+ Misses       4062     4059       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <100%> (+0.01%)` | :arrow_up: |
| #single | `41.85% <42.85%> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/27037?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/27037/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2NvbmNhdC5weQ==) | `96.85% <ø> (+0.36%)` | :arrow_up: |
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/27037/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `96.09% <100%> (+0.09%)` | :arrow_up: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/27037/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `97.04% <100%> (+0.45%)` | :arrow_up: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/27037/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `94.61% <100%> (+0.23%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/27037/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `88.88% <0%> (-11.12%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/27037/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.89% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27037?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27037?src=pr&el=footer). Last update [f0919f2...2b2a6fd](https://codecov.io/gh/pandas-dev/pandas/pull/27037?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @jbrockmendel 