(Tech Debt) Remove the course topic association with courses/Course model

[arslanashraf7]

Steps to Reproduce

In #2609, We shifted the association of topics courses/Course to cms/CourseProductPage to make the topics feat more robust and flexible to change. While we did that, we intentionally didn't remove the topics association that we had in courses/Course model because of the following reasons:

• We needed this to migrate the existing Course/CourseTopic association towards cms/CourseProductPage
• Removing the existing association would've made the PR/feat more complex and less flexible to test
• There was a parallel API feat(feat: update APIs to support external courseware data and additional fields #2608) in progress that needed the topics association as well, which made it hard to remove right away

Now that, we have launched the topics thing, we can easily remove the CourseTopic association from the Course model in the courses app. We can also easily test the change and refactor any things needed for this.

Acceptance Criteria

• Remove the courses/Course model association with courses/CourseTopic
• Refactor any usages accordingly, e.g removing tests and code references if any
• Make sure the topics functionality and the courses APIs keeps on working w.r.t topics

[arslanashraf7]

Closing, Deployed on prod via #2636 2649