TaskTree: Prepare for de-utils-ization - part 2

Move TaskTree into Tasking namespace.
Move Tasking namespace out of Utils namespace.

Change-Id: Ib4c1d7f54f1808517e54768dfa27209c33517b61
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/remotelinux/killappstep.cpp

@@ -15,8 +15,8 @@
 #include <utils/qtcassert.h>
 
 using namespace ProjectExplorer;
+using namespace Tasking;
 using namespace Utils;
-using namespace Utils::Tasking;
 
 namespace RemoteLinux::Internal {