Prevent copying SimpleDMatrix. (#5453)

* Set default dtor for SimpleDMatrix to initialize default copy ctor, which is
deleted due to unique ptr.

* Remove commented code.
* Remove warning for calling host function (std::max).
* Remove warning for initialization order.
* Remove warning for unused variables.

src/common/hist_util.h

@@ -297,9 +297,9 @@ struct Index {
   std::vector<uint8_t> data_;
   std::vector<uint32_t> offset_;  // size of this field is equal to number of features
   void* data_ptr_;
-  uint32_t* offset_ptr_;
-  size_t p_;
   BinTypeSize binTypeSize_;
+  size_t p_;
+  uint32_t* offset_ptr_;
   Func func_;
 };