API/delete-cohort

## ✨ Description
We need a Delete-mapping in CohortController, so the frontend gets an endpoint for deleting cohorts. The deletion of a cohort should result in correlated students and teachers having a null-value for cohort.

## 🎯 Use Case
To delete a cohort as a teacher/admin.

## 💡 Proposed Solution
Add a DeleteMapping to CohortController.

