ProjectExplorer: Use std::function for SimpleTargetRunner::start()

This spares us the typical r = runnable(); modify(r); setRunnable(r)
roundtrip and the m_runnable storage that might or might not
be the same as runControl->runnable. Similar for m_device.

Change-Id: I8300260dd8dd7cd395e40bcd3d2ae45089085008
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/debuggerruncontrol.h

@@ -166,15 +166,10 @@ public:
 
     ~GdbServerRunner() override;
 
-    void setRunnable(const ProjectExplorer::Runnable &runnable);
     void setUseMulti(bool on);
     void setAttachPid(Utils::ProcessHandle pid);
 
 private:
-    void start() override;
-
-    GdbServerPortsGatherer *m_portsGatherer;
-    ProjectExplorer::Runnable m_runnable;
     Utils::ProcessHandle m_pid;
     bool m_useMulti = true;
 };