Bluetooth: GATT: Turn GATT notify multiple feature default off

Turn the GATT notify multiple feature off as default value.
This feature changes the behavior of the bt_gatt_notify API in a way
that might not be backwards-compatible.
This is because the notify multiple header is larger, and therefore
limits the amount of bytes that could otherwise have been sent in a
normal notify PDU for a given ATT MTU.

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