Drop CUDACLANG target

This MR drops the CUDACLANG target. The CUDACLANG targets used clang to compile CUDA, but this MR drops it because:

• The latest supported version of CUDA that clang supports is 10.1, which is more than a year old.
• Early tests with CUDACLANG were about 20% slower.
• We don't currently have CUDACLANG builds as part of Allen's CI.