Remove code from JSON monitor for commands that won't be ported

Remove code from JSON monitor for commands that won't be ported

The QEMU developers have stated that they will not be porting
the commands 'pci_add', 'pci_del', 'usb_add', 'usb_del' to the
JSON mode monitor, since they're obsoleted by 'device_add'
and 'device_del'. libvirt has (untested) code that would have
supported those commands in theory, but since we already use
device_add/del where available, there's no need to keep the
legacy stuff anymore.

The text mode monitor keeps support for all commands for sake
of historical compatability.

* src/qemu/qemu_monitor_json.c: Remove 'pci_add', 'pci_del',
  'usb_add', 'usb_del' commands