SSH config UI: use "delete" consistently, avoid Yes/No buttons

"Delete" instead of "Remove" makes it more obvious the data is gone
afterwards, not just removed from a list.

Also use KMessageBox::warningYesNo instead of the helper method
KMessageBox::messageBox.
And fix missing i18n markup of menu entry.

Yes/No buttons in dialogs are discouraged (not only) by KDE HIG in favour
of actions terms.