replace PrepareMetalShaders with pre-build step

[davidkoski]

Consider moving PrepareMetalShaders into:

• https://github.com/ml-explore/mlx-swift/blob/main/tools/update-mlx.sh

It is doing similar tasks for JIT compiled shaders right now. The PrepareMetalsShaders plugin does the same for pre-built shaders, so it might be nice to do them in the same spot. It would also remove issues around the plugin approvals, see ml-explore/mlx-swift-examples#143