Remove HasCast and fix packetmath cast tests.

The use of the `packet_traits<>::HasCast` field is currently inconsistent with
`type_casting_traits<>`, and is unused apart from within
`test/packetmath.cpp`. In addition, those packetmath cast tests do not
currently reflect how casts are performed in practice: they ignore the
`SrcCoeffRatio` and `TgtCoeffRatio` fields, assuming a 1:1 ratio.

Here we remove the unsed `HasCast`, and modify the packet cast tests to
better reflect their usage.

unsupported/test/cxx11_tensor_morphing.cpp

@@ -53,7 +53,6 @@ static void test_static_reshape() {
   Eigen::IndexList<type2index<2>, type2index<3>, type2index<7>> dim;
   Tensor<float, 3> reshaped = tensor.reshape(static_cast<Eigen::DSizes<ptrdiff_t,3>>(dim));
 
-
   for (int i = 0; i < 2; ++i) {
     for (int j = 0; j < 3; ++j) {
       for (int k = 0; k < 7; ++k) {