refactor: remove TypedField's move constructor (XRPLF#4567)

Apply a minor cleanup in `TypedField`:
* Remove a non-working and unused move constructor.
* Constrain the remaining constructor to not be overly generic enough as
  to be used as a copy or move constructor.

src/ripple/protocol/SField.h

@@ -273,13 +273,7 @@ struct TypedField : SField
     using type = T;
 
     template <class... Args>
-    explicit TypedField(Args&&... args) : SField(std::forward<Args>(args)...)
-    {
-    }
-
-    TypedField(TypedField&& u) : SField(std::move(u))
-    {
-    }
+    explicit TypedField(private_access_tag_t pat, Args&&... args);
 };
 
 /** Indicate std::optional field semantics. */

src/ripple/protocol/impl/SField.cpp

@@ -38,6 +38,13 @@ struct SField::private_access_tag_t
 
 static SField::private_access_tag_t access;
 
+template <class T>
+template <class... Args>
+TypedField<T>::TypedField(private_access_tag_t pat, Args&&... args)
+    : SField(pat, std::forward<Args>(args)...)
+{
+}
+
 // Construct all compile-time SFields, and register them in the knownCodeToField
 // database: