Cleanup: Replace modifyVolume with modifyGeometrySet

This allows us to remove a callback from the modifier type info struct.
In the future the these modifiers might just be replaced by nodes
internally anyway, but in the meantime it's nice to unify the handling
of evaluated geometry a bit.

Differential Revision: https://developer.blender.org/D11080

source/blender/modifiers/intern/MOD_meshdeform.c

@@ -644,7 +644,6 @@ ModifierTypeInfo modifierType_MeshDeform = {
     /* modifyMesh */ NULL,
     /* modifyHair */ NULL,
     /* modifyGeometrySet */ NULL,
-    /* modifyVolume */ NULL,
 
     /* initData */ initData,
     /* requiredDataMask */ requiredDataMask,