MAINT remove deprecated function

[glemaitre]

Remove functions that have been deprecated for 2 versions.

[codecov]

Codecov Report

Merging #888 (9fd16e4) into master (16eda28) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #888      +/-   ##
==========================================
- Coverage   97.30%   97.30%   -0.01%     
==========================================
  Files          97       97              
  Lines        6389     6337      -52     
  Branches      727      712      -15     
==========================================
- Hits         6217     6166      -51     
+ Misses        103      102       -1     
  Partials       69       69              

Impacted Files	Coverage Δ
...mpling/_prototype_generation/_cluster_centroids.py	100.00% <ø> (ø)
...ototype_generation/tests/test_cluster_centroids.py	100.00% <ø> (ø)
imblearn/utils/deprecation.py	80.00% <ø> (ø)
imblearn/utils/testing.py	98.55% <ø> (-0.28%)	⬇️
imblearn/utils/tests/test_testing.py	100.00% <ø> (ø)
imblearn/metrics/tests/test_classification.py	100.00% <100.00%> (ø)
imblearn/utils/tests/test_deprecation.py	100.00% <100.00%> (ø)
imblearn/utils/tests/test_validation.py	100.00% <100.00%> (ø)
imblearn/datasets/tests/test_zenodo.py	91.89% <0.00%> (+2.70%)	⬆️

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