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/plugins/languageclient/currentdocumentsymbolsrequest.h

@@ -47,7 +47,7 @@ public:
     void start() final;
 };
 
-} // namespace LanguageClient
+using CurrentDocumentSymbolsRequestTask
+    = Tasking::CustomTask<CurrentDocumentSymbolsRequestTaskAdapter>;
 
-TASKING_DECLARE_TASK(CurrentDocumentSymbolsRequestTask,
-                        LanguageClient::CurrentDocumentSymbolsRequestTaskAdapter);
+} // namespace LanguageClient