dmaengine: ioat: remove unused ‘is_raid_device’

In ioat3_dma_probe(), variable ‘is_raid_device’ is initialized but never
used, which leads to warning with W=1

drivers/dma/ioat/init.c: In function ‘ioat3_dma_probe’:
drivers/dma/ioat/init.c:1084:7: warning: variable ‘is_raid_device’ set
but not used [-Wunused-but-set-variable]
  bool is_raid_device = false;

So remove it.

Cc: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>