Blender Git Loki

Cleanup: remove OMP's 'critical' sections in BKE_pbvh_node_add_proxy/free_proxies.

Not so useful now that we use BLI_task! Not sure why those were ever added actually,
readng carefully that code only modified data here is the PBVHNode, which is only
used/affected by one thread at a time ever. And shared read data (PBVH itself) is
not modified during brush execution itself, so it's safe to use it threaded too.