[Qt] remove broken OptionsModel::Upgrade()

When starting bitcoin-qt without a qt-settings file, strOverriddenByCommandLine contains some false values,
because OptionsModel::Upgrade() calls OptionsModel::Init() a second time and SoftSetArgs have been set before with default values.

Also to me it looks like OptionsModel::Init() assumes that AppInit2 is called after,
but OptionsModel::Upgrade() is called after AppInit2. This would mean that some options set in OptionsModel::Upgrade() are not even applied.
For example setting the proxy options through Upgrade() is broken, I assume (havent tested).

So my suggestion is to remove OptionsModel::Upgrade() completely.