signond: don't use qDeleteAll for children
signond: don't use qDeleteAll for children

The previous code was still causing crashes:
https://errors.ubuntu.com/problem/db0f212d945d95cdb0210cecaf2c4eff281f3815

The likely reason is that the compiler was optimizing out the list copy
(given that QObject::children() is const and qDeleteAll() takes a const
list reference as parameter).
See also https://bugs.kde.org/show_bug.cgi?id=284989 (though that is not
very helpful in explaining the issue).

plotters/accounts-sso.signond,plotters/accounts-sso.signond,plotters/accounts-sso.signond