[clang] [OpenCL][NVPTX] Don't set calling convention for OpenCL kernel (PR #170170)

[Shilei Tian via cfe-commits]

https://github.com/shiltian commented:

This doesn't seem like a right fix. To me it seems like `getDeviceKernelCallingConv` should not return the wrong CC in the first place.

https://github.com/llvm/llvm-project/pull/170170