Geometry Nodes: Revert current normal attribute implementation

After further thought, the implementation of the "normal" attribute
from D10541 is not the best approach to expose this data, mainly
because it blindly copied existing design rather than using the
best method in the context of the generalized attribute system.

In Blender, vertex normals are simply a cache of the average normals
from the surrounding / connected faces. Because we have automatic
interpolation between domains already, we don't need a special
`vertex_normal` attribute for this case, we can just let the
generalized interpolation do the hard work where necessary,
simplifying the set of built-in attributes to only include the
`normal` attribute from faces.

The fact that vertex normals are just a cache also raised another
issue, because the cache could be dirty, so mutex locks were
necessary to calculate normals. That isn't necessarily a problem,
but it's nice to avoid where possible.

Another downside of the current attribute naming is that after the
point distribute node there would be two normal attributes.

This commit reverts the `vertex_normal` attribute so that
it can be replaced by the implementation in D10677.

Differential Revision: https://developer.blender.org/D10676

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