Windows 10 will actually remove the Windows.old folder after 30 days to clear up space automatically according to multiple sources. You will not be able to delete it manually though...only the Disk Cleanup utility will be able to properly remove the directory after clicking the "Clean System files" button at the bottom of that window. I'm not sure if the Disk Cleanup applet will remove that BCD entry though, but you can highlight that entry and click on the delete button to clear that entry out and you should be good to go.