Improvement: Allow steps to be executed even though course no longer exists

[ScottVerbeek]

Use case: I want to clear up my database tables. After a course is backed-up, the backup file contains the logs that are in {logstore_standard_log} table. I then delete the course and finally I wish to remove all records from the {logstore_standard_log} table. I also don't want the deletion logs in the table, thus running the custom step (see below) before deletion isn't a option.

Here is the use case in a very simple plugin step: https://github.com/catalyst/moodle-tool_lcprunecourselogstep/blob/main/classes/lifecycle/step.php (plugin depends on #189 )