Hi Maan,
I believe that you have to reload, in order to free the IDB for this interface/subinterface and remove it entirely.
An Interface Descriptor Block (IDB) is a special control structure internal to the Cisco IOS software that contains information such as the IP address, interface state, and packet statistics. Cisco IOS software maintains one IDB for each interface present on a platform and one IDB for each subinterface.
BR,
Mohammed Mahmoud.