Remove C++17 fallbacks
With !188 (merged), IPPL has become C++20-dependent in order to avoid drastically unreadable code and clutter. Thus, the rest of the codebase is free to assume that C++20 features are available. This makes all C++17/C++20 checks obsolete. This MR removes all redundant C++17 versions of features and restores ParameterList printing.