Remove test for very low-value feature blocking PyCLIF-pybind11 work.

Using `unique_ptr` to pass an enum type is a very artificial construct.

Supporting this feature in PyCLIF-pybind11 is possible, but not worth the effort, including the effort for maintaining the added complexity.

It seems very unlikely that this feature is used in client code, and if it is, it will be best to change the client code to pass the enum directly (less generated machine code, more performant).

Tested:

```
tap_presubmit -p clif.pybind11 --detach --email -c 495090196
```

* http://tap/OCL:495090196:BASE:495090752:1670962252251:52d68751

PiperOrigin-RevId: 495125268

Author: rwgk

clif/testing/python/t4.clif

@@ -18,7 +18,6 @@ from "clif/testing/t4.h":
   def `walk_ast` as Walk(pb: AST) -> int
   def `size_ast` as Size(pb: AST) -> int
   def `decl_type` as DeclType(pb: Decl) -> Decl.Type
-  def `decl_type_uniq_out` as DeclTypeUO(pb: Decl) -> Decl.Type
   def `decl_type_uniq_in` as DeclTypeUI(pb: Decl) -> Decl.Type
   def all_ast_borrowed() -> list<AST>
   def all_ast_holds() -> list<AST>

clif/testing/python/t4_test.py

@@ -34,7 +34,6 @@ def testProtoNestedParam(self):
     pb.decltype = pb.FUNC
     self.assertEqual(t4.DeclType(pb), pb.FUNC)
     self.assertEqual(t4.DeclTypeUI(pb), pb.FUNC)
-    self.assertEqual(t4.DeclTypeUO(pb), pb.FUNC)
 
   def testProtoNestedMessage(self):
     pb = nested_pb2.Outer.Inner()

clif/testing/t4.h

@@ -38,9 +38,6 @@ inline Decl::Type decl_type(const Decl& pb) { return pb.decltype_(); }
 inline Decl::Type decl_type_uniq_in(std::unique_ptr<Decl> t) {
   return t->decltype_();
 }
-inline std::unique_ptr<Decl::Type> decl_type_uniq_out(const Decl& pb) {
-  return std::make_unique<Decl::Type>(pb.decltype_());
-}
 
 inline long size_any_ref(const ::proto2::Message& pb) {  // NOLINT
   return pb.ByteSizeLong();