CLN: Remove never-True Block.is_sparse

[jbrockmendel]

Along with some non-reachable branches in Block.quantile. This in turn lets us get rid of Block._na_value.

[pep8speaks]

Hello @jbrockmendel! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

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

Comment last updated at 2019-06-25 14:21:32 UTC

[codecov]

Codecov Report

Merging #27037 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            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%)	⬆️
#single	41.85% <42.85%> (-0.09%)	⬇️

Impacted Files	Coverage Δ
pandas/core/internals/concat.py	96.85% <ø> (+0.36%)	⬆️
pandas/core/internals/managers.py	96.09% <100%> (+0.09%)	⬆️
pandas/core/dtypes/concat.py	97.04% <100%> (+0.45%)	⬆️
pandas/core/internals/blocks.py	94.61% <100%> (+0.23%)	⬆️
pandas/io/gbq.py	88.88% <0%> (-11.12%)	⬇️
pandas/core/frame.py	96.89% <0%> (-0.12%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f0919f2...2b2a6fd. Read the comment docs.

[jreback]

thanks @jbrockmendel