folderView: Reset schemas before removing the folder

folderView: Reset schemas before removing the folder

When removing the last icon of a folder, FolderView first removes
the folder from org.gnome.desktop.app-folders.folder-children, then
proceeds to reset all its keys, which removes the relocatable schema.

That order of operations turns out to be problematic. Removing the
folder from 'folder-children' destroys the folder icon, which in turn
destroys the folder view, which throws a load of warnings in the
journal.

Fix that by removing the folder after resetting the schema keys. In
fact, what we're doing here is not using 'this' anymore.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/841