Remove deprecated compute_on_step in Regression

[vumichien]

What does this PR do?

Part of #956

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• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #967 (1fe59e5) into master (23b2e45) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #967   +/-   ##
=====================================
- Coverage      95%    95%   -0%     
=====================================
  Files         177    177           
  Lines        7509   7509           
=====================================
- Hits         7137   7124   -13     
- Misses        372    385   +13