[llvm] [NVPTX] Rip out vestigial variadic support (NFC) (PR #202385)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> LGTM in general.
> 
> Do we happen to recreate generated PTX exactly identical to what we're generating now? Or do we miss the test coverage?

The ExpandVariadics pass is totally superseding this support I believe. NVPTXISelLowering and NVPTXAsmPrinter will never see variadic functions today so I think this is NFC. 

https://github.com/llvm/llvm-project/pull/202385