remove INDEX file generation for 10.x

INDEX file was needed to make pkg_upgrade works. however, we use pkg(8)
to manage packages. pkg(8) is far better than pkg_* and pkg_upgrade in
bsdadminscripts, does not use INDEX file at all. some other tools like
ports-mgmt/psearch still use INDEX file, but it's too heavy to create
INDEX and we do not have a plan to use ports. disable it until we have
to.