Fix #12388: Vehicle::CopyVehicleConfigAndStatistics not releasing unit number

[JGRennison]

Motivation / Problem

#12388
Vehicle::CopyVehicleConfigAndStatistics does not release the current unit number before move-assigning the unit number of the source vehicle

Description

Fix the above

Limitations

N/A

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker