[SCSI] sd: remove command-size switching code

This patch (as1138) removes from sd.c some old code for switching from
10-byte commands to 6-byte commands.  This code is redundant -- the
switching for READ and WRITE is already handled in
scsi_io_completion() and the switching for MODE SENSE is already
handled in scsi_mode_sense().  (There is no comparable switch for MODE
SELECT, but I doubt one is needed.)

Furthermore the other handlers do a better job; they check for
appropriate ASC and ASCQ values before blindly switching the size.
The code in sd.c is known to cause problems with some devices by
switching when it shouldn't.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>