debugger: shutdown debugger when switching sessions

The code was already there, but disabled as it led to a crash during
Creator startup due to a spurious "sessionUnload" emission.

src/plugins/debugger/debuggermanager.cpp

@@ -1048,9 +1048,7 @@ void DebuggerManager::sessionLoaded()
 
 void DebuggerManager::sessionUnloaded()
 {
-    return;
-    //FIXME: Breakview crashes on startup as there is
-    //cleanupViews();
+    cleanupViews();
     if (m_engine)
         m_engine->shutdown();
     setStatus(DebuggerProcessNotReady);