Gnc-Prefs: migrate account_separator preference (and associated GtkEntry widget)

All GtkEntries now use the new preferences backend
so drop the GtkEntry-Gconf wiring as well.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23243 57a11ea4-9604-0410-9ed3-97b8803252fd