bug#27999: 26.0.50; delete-other-windows deletes side windows

[Alex]

I believe this is due to commit b8fd71d57 (C-x 1 works fine when
checking out its parent commit).

I tested using the following:

(display-buffer-in-side-window (get-buffer "*Messages*") '((side .
left)))

Executing "C-x 1" in Emacs 25.2 doesn't delete the *Messages* buffer,
but in master it does.