Drop c++14 compatibility in HltEventLoopMgr and StreamTagMakerTool
Since we now have C++17 available by default in athena/master builds, I'm removing the c++14 special cases from HltEventLoopMgr and StreamTagMakerTool.
From code of the form:
#if c++17 available
doSomethingModern();
#else
oldStandardWorkaround();
#endif
this MR is removing the if/else and the old-standard compatibility code.