Make endpoints which don't return anything yield NoContent

Relevant issue: OpenAPITools#9901

The haskell-http-client generator tries to generate a polymorphic return
type for endpoints which don't return anything in the success case, but
still produce content in other cases. This means that these endpoints
hit a decoding error in the success case, because there is no content to
decode.

This changes the behaviour so that endpoints that don't return anything
are *always* generated as returning NoContent, and never try to decode
the response. This change is based on a similar one for the
haskell-servant generator, which can be found at:

OpenAPITools#9830

which resolved a similar issue for that generator.

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/HaskellHttpClientCodegen.java

@@ -879,15 +879,8 @@ public boolean isDataTypeBinary(final String dataType) {
     private void processReturnType(CodegenOperation op) {
         String returnType = op.returnType;
         if (returnType == null || returnType.equals("null")) {
-            if (op.hasProduces) {
-                returnType = "res";
-                op.vendorExtensions.put(VENDOR_EXTENSION_X_HAS_UNKNOWN_RETURN, true);
-            } else {
-                returnType = "NoContent";
-                if (!op.vendorExtensions.containsKey(VENDOR_EXTENSION_X_INLINE_ACCEPT)) {
-                    SetNoContent(op, VENDOR_EXTENSION_X_INLINE_ACCEPT);
-                }
-            }
+            returnType = "NoContent";
+            SetNoContent(op, VENDOR_EXTENSION_X_INLINE_ACCEPT);
         }
         if (returnType.contains(" ")) {
             returnType = "(" + returnType + ")";