GtkListBox: Remove refilter signal

GtkListBox: Remove refilter signal

This was added from https://bugzilla.gnome.org/show_bug.cgi?id=691979
where it was wanted in order to get a "all children filtered" kind
of row. However, the solution is not really generic enough, because
filtering can happen for other reasons than a full refilter (i.e. due
to a row change), and the partial fix you can get by counting children
after refilter can be achieved by manually doing it whenever you manually
called refilter anyway.

Its possible we should have some better solution for the "empty state"
though.