Fix cohort/rule deletion, resolves #737 (MOODLE_405_STABLE)

[marxjohnson]

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[marxjohnson]

I think the tests were failing due to #734. I have rebased the branch with the fix, so hopefully everything passes now.

[abias]

Hi @marxjohnson ,

thank you very much for backporting the patch from #739 to the 4.5 stable branch.
This PR seems to have been rebased incorrectly as it contains commits of mine as well.
But don't worry, I will simply close the PR and will backport the patch from main to 4.5 myself afterwards.

Cheers,
Alex