remove alternative C++ Queue implementation in neatogen

remove alternative C++ Queue implementation in neatogen

This code was written to provide two versions of the same API, one for C and one
for C++. This is unnecessary because the C API is already usable from C++.
Moreover, one can just use std::queue in C++. This code was not being used as we
only ever #include it from C files, so we just remove it to ease ongoing
maintenance.