Stop timing out payments automatically, requiring abandon_payment

Stop timing out payments automatically, requiring abandon_payment

When the `abandon_payment` flow was added there was some concern
that upgrading users may not migrate to the new flow, causing
memory leaks in the pending-payment tracking.

While this is true, now that we're relying on the
pending_outbound_payments map for `send_payment` idempotency, the
risk of removing a payment prematurely goes up from "spurious
retry failure" to "sending a duplicative payment", which is much
worse.

Thus, we simply remove the automated payment timeout here,
explicitly requiring that users call `abandon_payment` when they
give up retrying a payment.