vle: assign CXXFLAGS correctly

In previous version, the CXXFLAGS includes CFLAGS thus, it duplicates
some value.
vle: assign a default value to CMAKE_BUILD_TYPE

We assign RelWithDebInfo if CMAKE_BUILD_TYPE is empty.
devs: remove useless code when deleting model

The CompleteBagModel::delModel function can only be call by Executives
objects and Executives are called after classical models. So, we cleanup
the function and we add an assertion.
devs: fix segfault when deleting models