refactor: Repetitive inline constexpr removed as no longer needed

Not needed anymore as stated in cplusplus/draft#4601

test/static/custom_rep_test_min_impl.cpp

@@ -61,7 +61,7 @@ public:
 }  // namespace
 
 template<typename T>
-inline constexpr bool mp_units::is_scalar<min_impl<T>> = true;
+constexpr bool mp_units::is_scalar<min_impl<T>> = true;
 
 template<typename T, typename U>
 struct std::common_type<min_impl<T>, min_impl<U>> : std::type_identity<min_impl<std::common_type_t<T, U>>> {};