CUDA target is unmaintained

[Ralith]

See #1321 (comment), Rust-GPU/Rust-CUDA#106. Since its last update, unstable features used by Rust-CUDA have been removed, rendering it unbuildable with modern rustc, and preventing us from depending on new Rust features without breaking CUDA users.

We should port to an up-to-date CUDA environment (https://github.com/coreylowman/cudarc?) or drop support.

[Ralith]

Decision is to drop support: #1321 (comment)