[llvm] AMDGPU: Delete seemingly dead s_fmaak_f32/s_fmamk_f32 folding code (PR #140580)

[Mirko Brkušanin via llvm-commits]

mbrkusanin wrote:

Cases that this was supposed to cover can no longer happen in this part of the code due to https://github.com/llvm/llvm-project/pull/127563 and https://github.com/llvm/llvm-project/pull/127626.

https://github.com/llvm/llvm-project/pull/140580