ProjectExplorer: Drop KitAspect::defaultValue()

The functionality of this function overlapped with KitAspect::setup(),
leading to unclear responsibilities and resulting in bugs such as the
one fixed by 776d54e435.
Therefore, we drop the defaultValue() function, merging its
implementation with setup() where applicable.

Change-Id: Iefa9c3df8b76e97ddf9ad388516621f7ea6558d4
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/debuggerkitinformation.h

@@ -41,7 +41,6 @@ class DEBUGGER_EXPORT DebuggerKitAspect : public ProjectExplorer::KitAspect
 public:
     DebuggerKitAspect();
 
-    QVariant defaultValue(const ProjectExplorer::Kit *k) const override;
     QList<ProjectExplorer::Task> validate(const ProjectExplorer::Kit *k) const override
         { return DebuggerKitAspect::validateDebugger(k); }