Add `nodiscard` attribute to StatusCode (C++17)

this allow spotting unchecked StatusCodes at compile time instead of at run time.

I also fixed O(250) unchecked StatusCodes in Gaudi (in ~60 files), using the script $587.

With this we can remove StatusCodeSvc with the overhead it implies (@graven noted that removing ~StatusCode we will also save some time because the return will be done via registers).

This also remove the WARN_UNUSED macro introduced with !386 (merged) (redundant).