When calling bioq_first() to see if a queue is empty in bioq_disksort(),

don't save the return value as we won't use it.

Noticed by:	Coverity Prevent analysis tool
MFC after:	3 days

sys/kern/subr_disk.c

@@ -153,7 +153,7 @@ bioq_disksort(bioq, bp)
 	/*
 	 * If the queue is empty then it's easy.
 	 */
-	if ((bq = bioq_first(bioq)) == NULL) {
+	if (bioq_first(bioq) == NULL) {
 		bioq_insert_tail(bioq, bp);
 		return;
 	}