Fix unbuilt units not being destroyed in factories when mobile build orders are cancelled

[lL1l1]

Issue

You can do "OnStopBuild" with an unfinished unit by cancelling a BuildMobile order on the ground. This gets the factory locked up since there is no way to remove the unfinished unit.

Description of the proposed changes

Destroy unbuilt units in FactoryUnit.OnStopBuild

Testing done on the proposed changes

An unfinished unit no longer remains in the factory after the mobile build order is cancelled.
Copy-paste command for testing:

CreateUnitAtMouse('urb0301', 0,    0.00,    0.00, -0.00000)
ForkThread(function() 
WaitTicks(10) 
ConExecute('UI_SelectByCategory +nearest FACTORY CYBRAN TECH3')
import('/lua/ui/game/commandmode.lua').StartCommandMode('build', {name = 'url0303'}) 
end)

Checklist

• Changes are annotated, including comments where useful
• Changes are documented in the changelog for the next game version