showcompletionconditions should not be set when completion is not being tracked for the course

When completion tracking is disabled for the course, the Show completion conditions setting should be disabled.

On the form, it is being hidden when completion tracking is disabled, but somehow it is still being set.

Perhaps it should work as follows:

1. showcompletionconditions should be allowed to be null and not set a default value of 1.
2. On upgrade, we set "showcompletionconditions" to 1 for existing courses with completion tracking.
3. On course creation/update, when completion is not being tracked for the course:
   1. When creating a course, we should unset "showcompletionconditions" so it is null when it gets registered
   2. When updating a course, we should set "showcompletionconditions" to null.

So when the course gets edited and Completion tracking is set to "Yes", the "Show completion conditions" select element will display the site defaults setting for "showcompletionconditions".