Bluetooth: host: gatt: respect `BT_GATT_ENFORCE_SUBSCRIPTION`

A subscription check was still being done in the
gatt_notify_multiple_verify_params() fn, regardless of the value of
CONFIG_BT_GATT_ENFORCE_SUBSCRIPTION.

This could lead to getting this warning if BT_GATT_NOTIFY_MULTIPLE is
enabled, and the notifications not being sent to unsubscribed peers.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>