Remove two unused methods from ui_interface

- Remove unused ThreadSafeHandleURI from ui_interface
- Remove unused ThreadSafeAskFee from ui_interface
  - ThreadSafeAskFee is effectively unused. It is only called when the fAskFee parameter on SendMoney or SendMoneyToDestination is true, which never happens (it is always used in the default which is false). Remove it.