Bluetooth: Shell: Add gatt-unregister-service command

gatt-unregister-service can be used to remove the test service at
runtime causing service changed to be indicated:

00:1b:dc:07:31:88 (public)> gatt-unregister-service
[bt] [DBG] gatt_indicate: (0x0011e100) conn 0x0011d880 handle 0x0008
[bt] [DBG] bt_att_req_send: (0x0011e100) conn 0x0011d880 req 0x0011db00
[bt] [DBG] att_send_req: (0x0011e100) req 0x0011db00
Unregistering test vendor service

< ACL Data TX: Handle 3585 flags 0x00 dlen 11
      ATT: Handle Value Indication (0x1d) len 6
        Handle: 0x0008
          Data: 0a001000
> ACL Data RX: Handle 3585 flags 0x02 dlen 5
      ATT: Handle Value Confirmation (0x1e) len 0

Jira: ZEP-2225
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>