st: Remove obsolete scsi_tape.max_pfn

Its last user was removed 10 years ago, in commit
8b05b773

 ("[SCSI] convert st to use scsi_execute_async").

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Ewan D. Milne <emilne@redhat.com>
Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>