Remove unnecessary lo_lseek call in lo_open. Apparently there was once

Remove unnecessary lo_lseek call in lo_open.  Apparently there was once
a need for it back in the neolithic era, but it's certainly dead code in
any PG release we would recognize as such.  Since it forces an additional
network round trip to the backend, getting rid of it should provide some
small performance improvement for large-object-using clients.