entities: remove :description from MergeRequest

What does this MR do?

Removes a duplicate description key from API JSON objects.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

description is already exported via ProjectEntity and doesn't need to be listed in MergeRequest as well.

What are the relevant issue numbers?

N/A

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together