Remove unused CDataStream::SetType

The last use was removed in the previous commit.

src/streams.h

@@ -366,7 +366,6 @@ public:
         ::SerializeMany(*this, std::forward<Args>(args)...);
     }
 
-    void SetType(int n)          { nType = n; }
     int GetType() const          { return nType; }
     void SetVersion(int n)       { nVersion = n; }
     int GetVersion() const       { return nVersion; }