Debugger: Shuffle plugin setup code around

To get closer to the standard setup.

Change-Id: I969abb44393532f5dba95e643bd8ba6356f44d16
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/debugger/debuggerplugin.h

@@ -42,8 +42,6 @@ public:
     DebuggerPlugin();
     ~DebuggerPlugin() override;
 
-    static DebuggerPlugin *instance();
-
 private:
     // IPlugin implementation.
     bool initialize(const QStringList &arguments, QString *errorMessage) override;