rbd example: 'invalidate=false' no longer necessary

As of commit 903b2812e9c7, rbd does support cache invalidation.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/examples/rbd.fio b/examples/rbd.fio
index 9d73ca1..c6901f4 100644
--- a/examples/rbd.fio
+++ b/examples/rbd.fio
@@ -15,7 +15,6 @@
 clientname=admin
 pool=rbd
 rbdname=fio_test
-invalidate=0	# mandatory
 rw=randwrite
 bs=4k