[C++] Fix compiler error from deleted assignment operator (google#6036)…

… (google#6047)

* [C++] Fix compiler error from deleted assignment operator (google#6036)

The assignment operator of the `buf_` member is deleted, we cannot call it from the assignment operator of the `TableKeyComparator` struct.

=> Also delete the assignment operator of the `TableKeyComparator` struct (already private anyhow).

* [C++] Fix compiler error from deleted assignment operator (google#6036) - fix extraneous semicolon

The assignment operator of the `buf_` member is deleted, we cannot call it from the assignment operator of the `TableKeyComparator` struct.

=> Also delete the assignment operator of the `TableKeyComparator` struct (already private anyhow).

include/flatbuffers/flatbuffers.h

@@ -1940,10 +1940,7 @@ class FlatBufferBuilder {
     vector_downward &buf_;
 
    private:
-    TableKeyComparator &operator=(const TableKeyComparator &other) {
-      buf_ = other.buf_;
-      return *this;
-    }
+    FLATBUFFERS_DELETE_FUNC(TableKeyComparator &operator=(const TableKeyComparator &other))
   };
   /// @endcond