Fix windows being inappropriately maximized on launch

When a window is closed while maximized, we write a special string to the config file so that it gets restored in its maximized state. But we don't ever delete that thing when the window is un-maximized and closed again, causing the window to always be maximized when launched.

This MR is speculative because I am not able to reproduce the issue. @guoyunhe can you test to see if this fixes the issue for you?

BUG: 426813 FIXED-IN: 5.78