ide-tape: remove remaining pipeline functionality

The driver is using now solely its own request queue.

- tape->next_stage is always NULL so it is safe to remove
  all code depending on tape->next_stage != NULL

- this patch removes the last place which sets
  IDETAPE_FLAG_PIPELINE_ACTIVE in tape->flags

[bart: add above explanations]

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>