Remove compute_on_step from aggregation and tests

[SkafteNicki]

What does this PR do?

Fixes part of #956
Removes compute_on_step from aggregation + removes references to it in our testing setup

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[codecov]

Codecov Report

Merging #990 (d926c35) into master (452864b) will increase coverage by 24%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #990     +/-   ##
=======================================
+ Coverage      71%    95%    +24%     
=======================================
  Files         177    177             
  Lines        7522   7522             
=======================================
+ Hits         5323   7146   +1823     
+ Misses       2199    376   -1823