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

All GncPeriodSelects now use the new preferences backend
so drop the GncPeriodSelect-Gconf wiring as well.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23250 57a11ea4-9604-0410-9ed3-97b8803252fd