Some modifiers used `MOD_deform_mesh_eval_get` to make sure they had
a mesh to retrieve vertex groups from. But since curves don't support
vertex groups anyway, and since the curve to mesh conversion is handled
by the (legacy) curve object modifier stack anyway, this is confusing
and unnecessary. This shouldn't give any behavior changes, but some
deform modifiers on legacy curve objects might be faster if they used
to do the conversion.
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