kio_trash: refactor code

• General code clean-up
• Remove TrashImpl::mkdir(), it was added with an '#if 0' in 2004, and it looks like the functionality is already handled by other methods
• Clean-up findTrashDirectory:
  • the m_lastId logic has been gone for a long time, trash dir ID's are computed based on device major/minor to local devices and some heuristic for network mounts, so as to get unique ID's
  • don't use idForTrashDirectory() to get the id that we've just inserted in the trash directory maps

NO_CHANGELOG