client: Add SPDX License Identifier

client: Add SPDX License Identifier

This patch adds SPDX License Identifier and removes the license text.

-------------------------------------
       License            COUNT
-------------------------------------
 GPL-2.0-or-later     :      11

License: GPL-2.0-or-later
   client/adv_monitor.c
   client/adv_monitor.h
   client/advertising.h
   client/display.c
   client/main.c
   client/display.h
   client/agent.h
   client/gatt.c
   client/agent.c
   client/advertising.c
   client/gatt.h

Signed-off-by: Anuj Jain <anuj01.jain@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>