Remove all Houdini conditional macros prior to 16.5.

Here's one such example:

#if (UT_VERSION_INT >= 0x0e0000b0) // 14.0.176 or later

This also involves removing support for Houdini shipping with Boost (since 16.5, Houdini has not shipped with a Boost dependency):

#if UT_VERSION_INT < 0x10050000 // earlier than 16.5.0
namespace hboost = boost;
#endif

We recently voted to retain GEO_PrimVDB/GU_PrimVDB in the codebase, but I suggest we move away from using H12.5 macros for these for improved clarity.