Don't use TBB for OpenVDB->NanoVDB grid conversion.

nanovdb from openvdb 12 (newest version packaged on Gentoo) passes a
non-const Range& lambda to tbb::parallel_reduce in util/Reduce.h, which
modern oneTBB rejects, breaking the nanovdb::FpN instantiations in
cycles/util/nanovdb.cpp. Upstream builds against openvdb 13 where this
is fixed. Falling back to the serial path only slows down VDB volume
upload a little.

--- a/intern/cycles/util/nanovdb.h
+++ b/intern/cycles/util/nanovdb.h
@@ -10,7 +10,6 @@
 #  include <openvdb/openvdb.h>

 #  define NANOVDB_USE_OPENVDB
-#  define NANOVDB_USE_TBB

 #  include <nanovdb/NanoVDB.h>  // manages and streams the raw memory buffer of a NanoVDB grid.