dockerfiles/cleanup_and_update: remove force opt

This change removes the '-f' option from the
cleanup_and_update dockerfiles script in order
to prevent aggressive removal of in-development
docker images

Signed-off-by: Kelley Spoon <kelley.spoon@linaro.org>
Change-Id: Icf6778303081565eb2e99ec84c2eb695b14b9021
diff --git a/ci-dockerfiles-deployment/cleanup-update.sh b/ci-dockerfiles-deployment/cleanup-update.sh
index a0a3abe..7506527 100644
--- a/ci-dockerfiles-deployment/cleanup-update.sh
+++ b/ci-dockerfiles-deployment/cleanup-update.sh
@@ -11,4 +11,4 @@
 done
 
 echo cleaning up
-docker system prune -f
+docker system prune