Delete callbackYields

This is a duplicate of FirstArgIsYield, and doesn't add much value.
Simplify.

Change-Id: Icaf1a922b5e5063d5b674ff594dbd5ee10fce500
Signed-off-by: Ed Tanous <edtanous@google.com>
diff --git a/include/sdbusplus/asio/object_server.hpp b/include/sdbusplus/asio/object_server.hpp
index 756f53a..de0feeb 100644
--- a/include/sdbusplus/asio/object_server.hpp
+++ b/include/sdbusplus/asio/object_server.hpp
@@ -65,8 +65,7 @@
     utility::get_first_arg_t<utility::strip_first_arg_t<
         utility::decay_tuple_t<boost::callable_traits::args_t<T>>>>,
     message_t>;
-template <typename T>
-static constexpr bool callbackYields = FirstArgIsYield_v<T>;
+
 template <typename T>
 static constexpr bool callbackWantsMessage = FirstArgIsMessage_v<T> ||
                                              SecondArgIsMessage_v<T>;
@@ -547,7 +546,7 @@
 
         const std::string& nameItr = methodOrSignalNames_.emplace_back(name);
 
-        if constexpr (callbackYields<CallbackType>)
+        if constexpr (FirstArgIsYield_v<CallbackType>)
         {
             callbacksMethod_[name] =
                 std::make_unique<coroutine_method_instance<CallbackType>>(