MAINT: Remove assertIsNone from testing

[gfyoung]

Title is self-explanatory.

Partially addresses #15990.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@e501e1d). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #16102   +/-   ##
=========================================
  Coverage          ?   90.84%           
=========================================
  Files             ?      159           
  Lines             ?    50773           
  Branches          ?        0           
=========================================
  Hits              ?    46123           
  Misses            ?     4650           
  Partials          ?        0

Flag	Coverage Δ
#multiple	88.62% <ø> (?)
#single	40.34% <ø> (?)

Impacted Files	Coverage Δ
pandas/util/testing.py	79.19% <ø> (ø)

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[jreback]

thanks!