Some cleanup of vehicle code

[SurFlurer]

Summary

none

Purpose of change

1. There's one unnecessary copy of vehicle parts.
2. When I wrote vehicle prototype exporting feature, I added some redundant validation code that can be safely removed. The vpart_reference iterator already iterates through all parts in an order that they can be installed, so there's no need to rearrange vehicle parts at a certain mount point.

Describe the solution

1. Remove the unnecessary copy.
2. Remove the redundant code.

Describe alternatives you've considered

Testing

Tests passed. Exporting vehicle prototypes still works as expected.

Additional context