Remove unnecessary (and No-op) normal calculation when sculpting on top
of deformed coordinates. Examples are shape keys and deform modifiers.
On a 1 million face mesh, this saved 100ms per stroke update.
This function actually did nothing since cfa53e0fbe,
so that large improvement comes for free.
Conceptually this is correct because when sculpting on deformed
coordinates, we don't change the positions of the base mesh directly.
In the future it might be better to allocate a separate array for
normals when using deformed coordinates, but it's not clear that's
necessary yet.
bf0180d206