fold preparing inferior into starting inferior

after refactoring the trk adapter startup, the split was not necessary
any more. in fact, it seemed fairly arbitrary at this point.

src/plugins/debugger/gdb/attachgdbadapter.cpp

@@ -84,14 +84,6 @@ void AttachGdbAdapter::handleGdbError(QProcess::ProcessError error)
     shutdown();
 }
 
-void AttachGdbAdapter::prepareInferior()
-{
-    QTC_ASSERT(state() == AdapterStarted, qDebug() << state());
-    setState(InferiorPreparing);
-    setState(InferiorPrepared);
-    emit inferiorPrepared();
-}
-
 void AttachGdbAdapter::startInferior()
 {
     QTC_ASSERT(state() == InferiorStarting, qDebug() << state());