intf_Destroy(): use vlc_object_release() and a destructor instead

intf_Destroy(): use vlc_object_release() and a destructor instead

While reading this, you will find a bunch of:
  while (find (VLC_OBJECT_INTERFACE))
     release; release;

These are of course plain BUGS (which are neither introduced nor fixed
by this commit). Imagine, for instance, what happens if two threads run
the code above at the same time... they end up releasing the interface
once too many.