Bluetooth: host: Fix Service Changed not cleared from flash

Fix Service Changed configuration stored in flash not deleted in flash
when calling bt_gatt_clear from bt_unpair.
When clearing the check for "is bonded" should not be made. If the
bond information is already removed this check will fail.
When clearing the check for "modified" should not be made, clearing
is in itself a modification, and should always be made.

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