[MRG] TST remove double check of sciki-learn estimator

[glemaitre]

Reference Issue

What does this implement/fix? Explain your changes.

Any other comments?

[codecov]

Codecov Report

Merging #339 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #339      +/-   ##
=========================================
- Coverage    98.6%   98.6%   -0.01%     
=========================================
  Files          68      68              
  Lines        3948    3947       -1     
=========================================
- Hits         3893    3892       -1     
  Misses         55      55

Impacted Files	Coverage Δ
imblearn/utils/estimator_checks.py	95.8% <100%> (-0.03%)	⬇️

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