refactor: unused is_specialization_of_unit trait removed

src/core/include/mp-units/framework/unit_concepts.h

@@ -136,12 +136,6 @@ void is_unit_impl(const derived_unit<Expr...>*);
 
 void is_unit_impl(const one*);
 
-template<typename T>
-inline constexpr bool is_specialization_of_unit = false;
-
-template<symbol_text Symbol, auto... Args>
-inline constexpr bool is_specialization_of_unit<named_unit<Symbol, Args...>> = true;
-
 template<typename T>
 inline constexpr bool is_specialization_of_prefixed_unit = false;