Fix compilation on UE 5.8

[gapspt]

Removes duplicate definitions from VrmConvertModel.cpp that also exist in VrmConvertModel_Description.cpp

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[zuyi53]

Have you fix the VrmConvertModel_Description ?

[gapspt]

Have you fix the VrmConvertModel_Description ?

No changes needed there. The issue was duplicate symbols when building for 5.8. The VrmConvertModel_Description is basically a duplicate of VrmConvertModel, so I had only to exclude VrmConvertModel from the compilation, except for one single function.

An alternative could be to move that function also to VrmConvertModel_Description.
Another alternative would be to keep both implementations in the same file instead of having two files.

I chose the fix that required less changes in order for the review to be easier, but feel free to structure it better as you prefer.