Decide on the future of displacement maps

Displacement maps are not useful without tessellation. (or so it would look from the code base, the details are somewhat fuzzy)

The RHI port has no support for these maps (for a multitude of reasons, tessellation stage dependency is only one them).

Additionally, they are not demoed or tested in any form in 5.15 either; there is no examples, tests or Lancelot cases for them, therefore noone knows if they even work in 5.15. (consider also that seemingly unrelated features like the depth prepass or depth texture generation are heavily affected when a displacement map is present, so simple testing is not even sufficient, it needs to be tested in combinations with various other features in order to exercise all related code and shader paths)

Figure out what the future of this feature is. Perhaps it needs to be removed from the API?