Remove reentrancy guard

[fedgiac]

Removes reentrancy checks in preparation for the audit.

We introduced reentracy checks after this discussion.

There is no plan of implementing the order update function before the audit or before deploying the first version of the ETH flow contract. The order update function and its extra reentrancy risks was the main motivations for the reentrancy guard, so there is no reason to keep the guard until development of a new contract version with this feature starts.

Note that reentrancy between createOrder and deleteOrder does not help an attacker. No instructions after the reentrancy entry point read or write the storage of the contract, so calling a function while reentering would be equivalent to calling a function after the call to deleteOrder is finished.

Test plan

Existing tests.