libsa/zfs: remove unused variable

In vdev_init_from_label() we obtain the pool guid but don't use it.

Reviewed by:		tsoome, mav, imp
Differential Revision:	https://reviews.freebsd.org/D51909

Author: glebius

stand/libsa/zfs/zfsimpl.c

@@ -1165,13 +1165,11 @@ vdev_from_nvlist(spa_t *spa, uint64_t top_guid, uint64_t txg,
 static int
 vdev_init_from_label(spa_t *spa, const nvlist_t *nvlist)
 {
-	uint64_t pool_guid, top_guid, txg;
+	uint64_t top_guid, txg;
 	nvlist_t *vdevs;
 	int rc;
 
-	if (nvlist_find(nvlist, ZPOOL_CONFIG_POOL_GUID, DATA_TYPE_UINT64,
-	    NULL, &pool_guid, NULL) ||
-	    nvlist_find(nvlist, ZPOOL_CONFIG_TOP_GUID, DATA_TYPE_UINT64,
+	if (nvlist_find(nvlist, ZPOOL_CONFIG_TOP_GUID, DATA_TYPE_UINT64,
 	    NULL, &top_guid, NULL) ||
 	    nvlist_find(nvlist, ZPOOL_CONFIG_POOL_TXG, DATA_TYPE_UINT64,
 	    NULL, &txg, NULL) != 0 ||