Bluetooth: ATT: remove `BT_ATT_ENFORCE_FLOW`

Enforcing the peer's behavior is not strictly necessary. All the host
should do is make sure it is resilient to a spec-violating peer.

Moreover, a growing number of platforms were disabling the check, as the
spec allows "batching" HCI num complete packets events, stalling ATT RX.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
Co-authored-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>