Gnc-Prefs: migrate GncDateEdit widgets (and associated preferences)

All GncDateEdits now use the new preferences backend
so drop the GncDateEdit-Gconf wiring as well.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23251 57a11ea4-9604-0410-9ed3-97b8803252fd