[ML] Remove impossible move constructor/assignment (#866)

CLeafNodeStatistics specified a default move constructor
and move assignment operator.  However, these cannot be
generated because the class has a const reference member.
They were not called anywhere, so this change removes the
= default directives to avoid compiler warnings.

Author: David Roberts

include/maths/CBoostedTreeImpl.h

@@ -185,12 +185,10 @@ class MATHS_EXPORT CBoostedTreeImpl final {
 
         CLeafNodeStatistics(const CLeafNodeStatistics&) = delete;
 
-        CLeafNodeStatistics(CLeafNodeStatistics&&) = default;
+        // Move construction/assignment not possible due to const reference member
 
         CLeafNodeStatistics& operator=(const CLeafNodeStatistics&) = delete;
 
-        CLeafNodeStatistics& operator=(CLeafNodeStatistics&&) = default;
-
         //! Apply the split defined by \p split.
         //!
         //! \return Shared pointers to the left and right child node statistics.