MAINT: Remove testing.assert_isinstance

[gfyoung]

Deprecated in 0.17.0

xref #10458

[codecov-io]

Codecov Report

Merging #15652 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15652      +/-   ##
==========================================
- Coverage   91.03%   91.01%   -0.03%     
==========================================
  Files         143      143              
  Lines       49390    49388       -2     
==========================================
- Hits        44964    44951      -13     
- Misses       4426     4437      +11

Impacted Files	Coverage Δ
pandas/util/testing.py	80.88% <ø> (-0.04%)	❌
pandas/io/gbq.py	25% <0%> (-58.34%)	❌
pandas/tools/merge.py	93.61% <0%> (-0.34%)	❌
pandas/core/frame.py	97.87% <0%> (-0.1%)	❌

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[jreback]

thanks!