ixgbe: drop null test before destroy functions

Remove unneeded NULL test.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@ expression x; @@
-if (x != NULL)
  \(kmem_cache_destroy\|mempool_destroy\|dma_pool_destroy\)(x);
// </smpl>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Tested-by: Darin Miller <darin.j.miller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_fcoe.c

@@ -620,7 +620,6 @@ static void ixgbe_fcoe_dma_pool_free(struct ixgbe_fcoe *fcoe, unsigned int cpu)
 	struct ixgbe_fcoe_ddp_pool *ddp_pool;
 
 	ddp_pool = per_cpu_ptr(fcoe->ddp_pool, cpu);
-	if (ddp_pool->pool)
 	dma_pool_destroy(ddp_pool->pool);
 	ddp_pool->pool = NULL;
 }