This patch removes no-op node editor preview code (`PR_NODE_RENDER`)
and most calls to `BKE_node_preview_init_tree`. The only remaining call is
in the compositor.
- Shader nodes previews don't seem to do anything.
- In-node previews for the texture node system doesn't work either.
This is a first step to refactoring to remove `preview_xsize`,
`preview_ysize`, and `prvr` from nodes in DNA, aligned with
the general goal of removing runtime/derived data from data
structs.
Differential Revision: https://developer.blender.org/D13578
43875e8dd1