remove custom property and use standard sourceModel for our proxy model

remove custom property and use standard sourceModel for our proxy model

a QSortFilterProxyModel will trigger a new sort whenever needed, so
custom code is not needed

we might have someone that would use the standard property and loose teh
side effects of setting the source model via thet non standard property

this makes it more complicated to read as one might wonder why a custom
property was needed

deletes a bunch of code not really needed

Signed-off-by: Matthieu Gallien <matthieu.gallien@nextcloud.com>