Stop preventing users from deleting last board

Currently, we prevent users who have permission to delete boards from deleting the last board in a project or group in the UI, by simply not showing the delete button. This causes confusion for users who have this permission and expect to be able to delete this board without first going and creating another board so that the one they want to delete is no longer the last remaining board.

However, users with the right permission can still delete this board via the API. When a user does so, a default "Development" board is created so that there is at least 1 board.

In the UI, we should stop preventing users with the right permissions from deleting a board just because it's the last remaining one, since there is already logic to cover this situation and the user can already achieve it by other means.