chore: remove course/course topic association

[arslanashraf7]

Pre-Flight checklist

• Screenshots and design review for any changes that affect layout or styling
  • Desktop screenshots
  • Mobile width screenshots
• Migrations
  • Migration is backwards-compatible with current production code
• Testing
  • Code is tested
  • Changes have been manually tested
• Settings
  • New settings are documented and present in app.json
  • New settings have reasonable development defaults, if applicable

What are the relevant tickets?

#2647

What's this PR do?

• Removed the association between course and course topic models since we've moved that to course page, Take a look the ticket for more details
• refactors the code that used this association

How should this be manually tested?

• The api/courses and /api/programs should work as expected and return the proper topic values associated through CMS in the APIs
• Make sure the frontend e.g. topic filter works fine
• Just an overall smoke test

Where should the reviewer start?

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Any background context you want to provide?

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Screenshots (if appropriate)

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What GIF best describes this PR or how it makes you feel?

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[arslanashraf7]

@asadali145 I did some more cleanup in 01c16db. Could you take a look?

[asadali145]

LGTM!