Remove some dead code from the abi code (#4653)

These were originally used by the old backend framework as part of
legalizing function signatures for the respective ABI.

cranelift/codegen/src/isa/s390x/abi.rs

@@ -249,20 +249,6 @@ impl ABIMachineSpec for S390xMachineDeps {
         for i in 0..params.len() {
             let mut param = params[i];
 
-            // Validate "purpose".
-            match &param.purpose {
-                &ir::ArgumentPurpose::VMContext
-                | &ir::ArgumentPurpose::Normal
-                | &ir::ArgumentPurpose::StackLimit
-                | &ir::ArgumentPurpose::SignatureId
-                | &ir::ArgumentPurpose::StructReturn
-                | &ir::ArgumentPurpose::StructArgument(_) => {}
-                _ => panic!(
-                    "Unsupported argument purpose {:?} in signature: {:?}",
-                    param.purpose, params
-                ),
-            }
-
             let intreg = in_int_reg(param.value_type);
             let fltreg = in_flt_reg(param.value_type);
             let vecreg = in_vec_reg(param.value_type);