LibCore: Remove ObjectPtr in favor of RefPtr

Now that CObject is fully ref-counted, just use RefPtr everywhere! :^)

Applications/SystemMonitor/ProcessFileDescriptorMapWidget.h

@@ -14,6 +14,6 @@ public:
 private:
     explicit ProcessFileDescriptorMapWidget(GWidget* parent);
 
-    ObjectPtr<GTableView> m_table_view;
+    RefPtr<GTableView> m_table_view;
     pid_t m_pid { -1 };
 };