refactor: simplify async D-Bus connection handling

include/sdbus-c++/Message.h

@@ -56,7 +56,6 @@ namespace sdbus {
     class MethodReply;
     namespace internal {
         class ISdBus;
-        class IConnection;
     }
 }
 
@@ -258,12 +257,11 @@ namespace sdbus {
         bool doesntExpectReply() const;
 
     protected:
-        MethodCall(void *msg, internal::ISdBus* sdbus, const internal::IConnection* connection, adopt_message_t) noexcept;
+        MethodCall(void *msg, internal::ISdBus* sdbus, adopt_message_t) noexcept;
 
     private:
         MethodReply sendWithReply(uint64_t timeout = 0) const;
         MethodReply sendWithNoReply() const;
-        const internal::IConnection* connection_{};
     };
 
     class MethodReply : public Message