remove unreachable states, assert they are unreachable just in case

[jbrockmendel]

No luck so far in tracking down the issue where this was discussed.

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #19180 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #19180      +/-   ##
==========================================
+ Coverage   91.52%   91.52%   +<.01%     
==========================================
  Files         147      148       +1     
  Lines       48775    48776       +1     
==========================================
+ Hits        44639    44642       +3     
+ Misses       4136     4134       -2

Flag	Coverage Δ
#multiple	89.89% <100%> (ø)	⬆️
#single	41.61% <100%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/internals.py	94.49% <100%> (+0.06%)	⬆️
pandas/core/dtypes/cast.py	88.42% <0%> (-0.16%)	⬇️
pandas/tools/hashing.py	100% <0%> (ø)

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[jbrockmendel]

CI has been on the fritz for a few days. Should I keep trying or wait until further notice?

[jreback]

this is fine. generally if the CI fails for an othogonal reason to the PR don't keep pushing.