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.cpp

@@ -152,8 +152,6 @@ DebuggerKitAspect::DebuggerKitAspect()
     setPriority(28000);
 }
 
-QVariant DebuggerKitAspect::defaultValue(const Kit *) const { return QVariant(); }
-
 void DebuggerKitAspect::setup(Kit *k)
 {
     QTC_ASSERT(k, return);