[flang][OpenMP] Remove unused DECLARE REDUCTION from openmp-utils.h, NFC (#161390)

DECLARE REDUCTION is now handled by the generic code, and the special
handling no longer applies.

Author: kparzysz

flang/include/flang/Parser/openmp-utils.h

@@ -39,7 +39,6 @@ struct ConstructId {
   }
 
 MAKE_CONSTR_ID(OpenMPDeclarativeAllocate, D::OMPD_allocate);
-MAKE_CONSTR_ID(OpenMPDeclareReductionConstruct, D::OMPD_declare_reduction);
 MAKE_CONSTR_ID(OpenMPExecutableAllocate, D::OMPD_allocate);
 
 #undef MAKE_CONSTR_ID
@@ -92,7 +91,6 @@ struct DirectiveNameScope {
       if constexpr (std::is_base_of_v<OmpBlockConstruct, T>) {
         return std::get<OmpBeginDirective>(x.t).DirName();
       } else if constexpr (std::is_same_v<T, OpenMPDeclarativeAllocate> ||
-          std::is_same_v<T, OpenMPDeclareReductionConstruct> ||
           std::is_same_v<T, OpenMPExecutableAllocate>) {
         return MakeName(std::get<Verbatim>(x.t).source, ConstructId<T>::id);
       } else {