Allow soft delete of letter types

[christina-luong]

Describe the bug

An error pops up when deleting certain letter types.

Steps to reproduce

List any steps to reproduce behavior

1. Create a new letter type in Admin Portal and use it in a case (or find a letter type that has been used in a case previously).
2. Try to delete the letter type.
3. An error should pop up.

Expected behavior

When you delete a letter type, it should disappear from the Admin Portal. Letters that were created from this letter type should persist in the system.

Checklist

• Indicate bug severity on the right

Additional context