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

[Matt Arsenault via llvm-commits]

arsenm wrote:

### Merge activity

* **May 29, 5:28 PM UTC**: A user started a stack merge that includes this pull request via [Graphite](https://app.graphite.dev/github/pr/llvm/llvm-project/140580).


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