openvdb: remove openexr and ilmbase dependencies
Neither openexr nor ilmbase dependencies were actually being picked up
(checked with nix why-depends).
Instead, openvdb defaults to building its own vendored Half library since 8.1.0:
https://github.com/AcademySoftwareFoundation/openvdb/commit/5c9f0f9685805d883c187d9f04a8e8cff00e90e2
Also, starting from 11.0.0, OpenEXR 2 will no longer be supported.
Someone else can look into fixing this properly to use system libraries.