qemu: monitor: Remove JSON impls of drive_add and drive_del
    
    qemu won't ever add those functions directly to QMP. They will be
    replaced with 'blockdev-add' and 'blockdev-del' eventually. At this time
    there's no need to keep the stubs around.
    
    Additionally the drive_del stub in JSON contained dead code in the
    attempt to report errors. (VIR_ERR_OPERATION_UNSUPPORTED was never
    reported). Since the text impl does have the same message it is reported
    anyways.