core: Remove OrderExpirationBuffer config

[fabioberger]

Currently, we allow the operator to customize a buffer they'd like added to UTC time when validating an order's expiry. This buffer is applied when validating orders submitted via JSON-RPC as well as orders submitted from other peers. If peers have this set to different values, a peer with a lower buffer would consider the orders from a peer with a higher buffer invalid and penalize the peer for sharing those orders. We should either remove this configuration or alternatively, make it only apply to the JSON-RPC OrderEvents subscription, and not the validation performed when sharing orders with peers.