Because:
* Something like it did not exist before PR 107404
* That it is not run our mir-opt-level 0 indicates that it is not
required for soundness
* Its `MirPass::can_be_overridden()` is unchanged and thus returns true,
indicating that it is not a required MIR pass.
* No test fails in PR 151426 that stops enabling by default in non-optimized builds
As can be seen from the updated test `tests/mir-opt/optimize_none.rs`,
this means that `#[optimize(none)]` functions become even less
optimized. As expected and as desired.
