Blender Git Loki

Nodes: Improve performance when freeing a node tree

This commit makes freeing a node tree about 25 to 30 times faster.
Freeing a node tree happens whenever it is edited. Freeing a node
tree with about 4000 nodes went from 30-50ms to about 2 ms.

This was so slow before because for every node that was freed
when freeing the node tree, `node_free_node` looped over all
other nodes to detach frames, and then looped over all links to
remove any links connected to the node. That was all pointless
work because everything else is about to be freed anyway.

Instead, move that "detaching" behavior to the dedicated function
for removing a single node, and to the "local" version of the free
function to be safe, since I know less about what that version expects.

Differential Revision: https://developer.blender.org/D13636