Remove default value from AccountService::value()
Remove default value from AccountService::value()

This allows one to simply call

   v = accountService->value(QString("key"));

which was previously resulting in a compiler error since that call was
ambiguous (because there are two method with the same name and the first
QString parameter, and having all the other parameters as optional).

This change is API and ABI safe.

plotters/accounts-sso.libaccounts-qt,plotters/accounts-sso.libaccounts-qt