The node level was an indication of how deep the node was in the tree.
It was only used for detecting link cycles. Now that the node topology
cache from 25e307d725 exists, this calculation can be removed
completely.
The level calculation was quadratic and very slow on larger node trees.
In the mouse house file with a few thousand nodes, it took 23ms on
every single update. Another benefit is storing slightly less runtime
data, though this was only 2 bytes per node.
Differential Revision: https://developer.blender.org/D16566
be1745425c