Fix lots of warnings of -Wdeprecated-copy
C++11 provides default copy assignment operators, so manual implementations of operator= now is deprecated and unnecessary (https://en.cppreference.com/w/cpp/language/copy_assignment).
C++11 provides default copy assignment operators, so manual implementations of operator= now is deprecated and unnecessary (https://en.cppreference.com/w/cpp/language/copy_assignment).