cleanup: remove copy contructors when the compiler is able to generate a satisfactory

default copy constructor; remove useless static_cast's; some misc cleanup.

Eigen/src/Core/Difference.h

@@ -41,9 +41,6 @@ template<typename Lhs, typename Rhs> class Difference : NoOperatorEquals,
       assert(lhs.rows() == rhs.rows() && lhs.cols() == rhs.cols());
     }
 
-    Difference(const Difference& other)
-      : m_lhs(other.m_lhs), m_rhs(other.m_rhs) {}
-
   private:
     static const int _RowsAtCompileTime = Lhs::RowsAtCompileTime,
                      _ColsAtCompileTime = Rhs::ColsAtCompileTime;