[SCSI] st: remove unused st_buffer.in_use

I noticed that in_use in st_buffer is not used. The patch below
against 2.6.17-rc3 removes it, assuming there is no future use for it.
It was tested in a sparc SS20 with a DLT4000.

Signed-off-by: Martin Habets <errandir_news@mph.eclipse.co.uk>
Acked-by: Kai Mäkisara <kai.makisara@kolumbus.fi>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>