`surfaceToPolygon` is only useful for `QgsCurvePolygon` which inherits
from `QgsSurface`. The next commit will introduce
`QgsPolyhedralSurface` which also inherits from `QgsSurface`. For
`QgsPolyhedralSurface`, it does not make sense to have
`surfaceToPolygon`.
By moving `surfaceToPolygon` definition in `QgsCurvePolygon` this
allows to keep the functionality.