Pull request: Fix withdrawn convoy waiting for schedule

[freddyhayward]

https://github.com/jamespetts/simutrans-extended/pull/408
This pull request fixes an issue where convoys arriving at a stop after being set to withdraw had to wait for schedule before being sold, sometimes for many hours. This is effectively a quality-of-life change with zero balance implications since those convoys can already be manually sold instantly using the sell button or re-toggling the withdraw button.

[jamespetts]

Thank you - now incorporated.