qemu: monitor: Remove JSON impls of drive_add and drive_del

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.