libio: Remove __libc_readline_unlocked

__nss_readline supersedes it.  This reverts part of commit
3f5e3f5d ("libio: Implement
internal function __libc_readline_unlocked").  The internal
aliases __fseeko64 and __ftello64 are preserved because
they are needed by __nss_readline as well.

Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>