SidebarRow: Unref CloudProvider on change/finalize

SidebarRow: Unref CloudProvider on change/finalize

It was never unref()d, either when replacing the existing GObject in
set_property(), cleaning up in finalize(), or becoming a placeholder.

Fix by using g_set_object() and g_clear_object() to unref as needed.

This also drops the check that the newly set object is a valid cloud
provider account, as we don’t do the equivalent for any of the other
object-typed properties, and Carlos didn’t think this was important.

https://bugzilla.gnome.org/show_bug.cgi?id=787600