megaraid_sas: Remove unused variables in megasas_instance

The following patch for megaraid_sas removes some unused variables from the megasas_instance structure.

Signed-off-by: Adam Radford <aradford@gmail.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>