Variant: Remove the now-unnecessary local g_variant_parse_error_quark().

This is in glib itself now:
https://git.gnome.org/browse/glib/commit/glib/gvariant.h?id=eeac91f866939fd033303f34c7eb9d2b57c769fb
and was causing segfaults.

Bug #727554 (Antoine Jacoutot)