Remove [[nodiscard]] in fixed_size_list::is_empty
Remove [[nodiscard]] in fixed_size_list::is_empty

The attribute isn't C++14, warns in Clang because of that, is not in the
public interface, and unlike empty() it's obvious that the function is
not modifying the collection, so it's not worth adding more macros for
that single function.

Morwenn/cpp-sort,Morwenn/cpp-sort,Morwenn/cpp-sort,Morwenn/cpp-sort