rq->cmd[0] is never set to REQ_IDETAPE_READ_BUFFER so remove
REQ_IDETAPE_READ_BUFFER handling from idetape_create_write_cmd()
and the define itself.

Then remove no longer used idetape_create_read_buffer_cmd()
and IDETAPE_RETRIEVE_FAULTY_BLOCK define.

There should be no functional changes caused by this patch.

Cc: Borislav Petkov <petkovbb@gmail.com>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>