Rollup merge of #101484 - oli-obk:no_zst, r=eddyb

Remove dead broken code from const zst handling in backends

cc `@RalfJung`

found by `@eddyb` in https://github.com/rust-lang/rust/pull/98957#discussion_r963744605

compiler/rustc_codegen_llvm/src/common.rs

@@ -226,10 +226,6 @@ impl<'ll, 'tcx> ConstMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         })
     }
 
-    fn zst_to_backend(&self, _llty: &'ll Type) -> &'ll Value {
-        self.const_undef(self.type_ix(0))
-    }
-
     fn scalar_to_backend(&self, cv: Scalar, layout: abi::Scalar, llty: &'ll Type) -> &'ll Value {
         let bitsize = if layout.is_bool() { 1 } else { layout.size(self).bits() };
         match cv {