[clang] [CUDA] Remove sema check of function declaration with variadic argument (PR #161350)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > Just remove the handling and keep the flag, it should be fine based off of what Artem said now that we handle this by default.
> 
> Yes, I think that's what this PR does.

Am I missing something, we still have `CUDAAllowVariadicFunctions`.

https://github.com/llvm/llvm-project/pull/161350