background: Remove uses of libsocialweb and BgFlickrSource

BgFlickrSource will be merged with BgPicturesSource and rewritten using
Grilo at the same time.

https://bugzilla.gnome.org/show_bug.cgi?id=707569
background: Minor clean up

https://bugzilla.gnome.org/show_bug.cgi?id=707569