Don't use g_slist_free_full().

It's only in GLib 2.28 or later, but we don't require 2.28.

Instead, have col_custom_field_ids_free() do what g_slist_free_full() does -
and more.

Change-Id: I15768cfd2d26c847470b02ea6c51830dabcaa7ee
Reviewed-on: https://code.wireshark.org/review/5894
Reviewed-by: Guy Harris <guy@alum.mit.edu>