Deleting lectures doesn't give immediate visual feedback to the user

[karjo24]

Describe the bug
When deleting lectures on the course admin page, the lectures are not immediately removed from the page. This might suggest to the user that it was not actually deleted and might lead them to retry. Retrying will fail with "An unknown error occured during the deletion process", probably as double deletion cannot occur. The lecture disappears when reloading the page.

To Reproduce
Steps to reproduce the behavior:

1. Go to course admin page
2. Click on 'Delete 1 Lecture'
3. Verify that the lecture has not disappeared
4. Click on 'Delete 1 Lecture' again
5. See error alert
6. Reload page and see that lecture has disappeared

Expected behavior
The lecture immediately disappears to avoid confusion.