python nodes on Blender exit.
The nodeFreeNode function is calling a customizable freefunc callback,
which can be implemented by python. However, the context is invalid when
using this callback while the node tree data block is freed on exit.
Luckily it is also not necessary: When freeing the bNodeTree data blocks
all that is needed is freeing of the DNA data, no other side effects
should happen. So now disable the api callbacks when freeing nodes in
the ntreeFreeTree function.
Also some minor cleanup: checking node->typeinfo is not necessary, all
nodes will always have a valid typeinfo pointer - if a node type is
unknown this will be a stub bNodeType to avoid the need for such checks.
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