Unconditionally use Order::loadForUpdate when processing orders

Verified manually that the webhookTransaction() is still working as expected using my postfinance checkout test account and hookdeck.

Leaving this on needs-review for a couple of days in order give other people a chance to verify themselves.

Current MR just removes the workaround and as a result fails locking the order during the payment update. It should retain the locking.

No need to change that at the moment.