deprecate no longer required concat_alternatives

lhcb/Rec!3599 (merged) removes the last (unnecessary!) use of concat_alternatives from LHCb code -- so it is time to flag this code as deprecated, to be followed with removal at some point in the future.

GaudiExamples/src/FunctionalAlgorithms/selector.cpp

@@ -20,9 +20,7 @@ namespace Gaudi {
         : public Functional::FilterPredicate<bool( const Gaudi::Range_<Gaudi::Examples::MyTrack::ConstVector>& )> {
     public:
       CountSelectedTracks( const std::string& name, ISvcLocator* pSvc )
-          : FilterPredicate( name, pSvc,
-                             { KeyValue{ "InputData", Functional::concat_alternatives( "BogusLocation", "MoreBogus",
-                                                                                       "MyOutTracks" ) } } ) {}
+          : FilterPredicate( name, pSvc, { KeyValue{ "InputData", "MyOutTracks" } } ) {}
 
       StatusCode initialize() override {
         StatusCode sc = FilterPredicate::initialize();