Bluetooth: GATT: Remove value_handle from bt_gatt_chrc

The spec says it should always be the very first descriptor after the
characteristic:

  "BLUETOOTH SPECIFICATION Version 4.2 [Vol 3, Part G] page 534:
   3.3.2 Characteristic Value Declaration
   The Characteristic Value declaration contains the value of the
   characteristic. It is the first Attribute after the characteristic
   declaration. All characteristic definitions shall have a
   Characteristic Value declaration."

Change-Id: I6c38dea9cc4c1a05997edbd348e2759680472725
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>