TaskTree: Get rid of registration macros

Alias task types manually.
Don't require the alias to be inside the Tasking namespace.
Addresses the 22th point of the jira ticket below.

Task-number: QTCREATORBUG-28741
Change-Id: I1bdda7fe5a01e4bcb5052ec328f4e0eace878651
Reviewed-by: hjk <hjk@qt.io>

src/libs/utils/process.h

@@ -222,8 +222,8 @@ public:
     void start() final;
 };
 
+using ProcessTask = Tasking::CustomTask<ProcessTaskAdapter>;
+
 } // namespace Utils
 
-TASKING_DECLARE_TASK(ProcessTask, Utils::ProcessTaskAdapter);
-
 #endif // UTILS_PROCESS_H