Remove Vulkan

As it stands Vulkan in iMSTK doesn't work. It really doesn't seem like we'll be able to realistically support this. We'd sooner support Unity or Unreal which can be used for more graphical performance/fidelity. If this is the case, some other 3rd party module of iMSTK should be used if anyone wants to develop this.

Not to mention VTK has been making progress in rendering fidelity. I'm confident we'll be able to write some custom shaders for it in the future. But there are a few hard limits to what VTK can do currently. These things can be done in Unity and Unreal though. Also worth mentioning there are few things that VTK can do that Unity/Unreal can't (without 3rd party or other bad solutions), namely Volume Rendering which would be particularly difficult to support on Vulkan.