It's not possible to clear CT description field value or description of CT's fieldtype.

Steps:

1. Go to e.g. Admin / Content Types / Content.
2. Edit Article CT.
3. Set Description field value.
4. Save.
5. Edit Article CT again.
6. Clear Description field value.
7. Save.
8. Repeat above steps for any fieldtype of this CT.

Actual result: Value is not cleared.
Expected result: It's possible to clear field value.