Bluetooth: samples: peripheral_dis: Failed to save with custom settings

The Peripheral Device Information Service (DIS) sample implements it's
own custom settings backend in order to load runtime settings.
This results in errors when the BT stack tries to save entries through
the custom handler since no save handler exists.
Error messages:
 - bt_settings: Failed to save ID (err -2)
 - bt_gatt: Failed to save Database Hash (err -2)

Since this is not a sample of how to do custom settings backend it is
best simply to remove using the custom backend, as it is not required
in order to load runtime settings.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>