Previously the UV unwrapping handling for subsurf modifiers used
`DerivedMesh`to implement the subdivision. Since we're trying to remove
`DerivedMesh` in general, and since this just made use of the `Mesh`
data anyway, it's relatively simple to remove it here. Combined with
D15939, this makes it possible to remove more `DerivedMesh` code.
Differential Revision: https://developer.blender.org/D16487
b2d9716b4a