fport_seek: Eliminate a fruitless use of 'off_t_or_off64_t'.

fport_seek: Eliminate a fruitless use of 'off_t_or_off64_t'.

This is a followup to commit 91ba73b397fcc2a36ae7e434522a924c7a8887d0.

* libguile/fports.c (fport_seek): Use 'lseek' instead of
'lseek_or_lseek64', and use 'scm_t_off' uniformly.  That's the type used
in the function signature, and there's no benefit to using a wider type
internally.  Remove the overflow check, which is no longer needed.