CLN cleanup stale code in when reducing a class

[pierreglaser]

As a consequence of #262, I think some past workarounds used when saving dynamic classes can be removed. All tests seem to pass.

[codecov]

Codecov Report

Merging #271 into master will decrease coverage by 0.15%.
The diff coverage is n/a.

@@ Coverage Diff @@## master #271 +/- ##
==========================================
- Coverage 89.35% 89.19% -0.16% 
==========================================
Files 1 1 Lines 620 611 -9 Branches 131 127 -4 ==========================================
- Hits 554 545 -9 
Misses 43 43 Partials 23 23

Impacted Files	Coverage Δ
cloudpickle/cloudpickle.py	89.19% <ø> (-0.16%)	⬇️

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[ogrisel]

Thanks very much @pierreglaser .