env/plan9: revert SMP changes, stop running cmd/go tests

Revert "env: enable SMP in Plan 9 image"
This reverts commit 6478d3e7bdff4c8d810ba44f970ca4b0bfc92405.

Revert "dashboard: bump plan9 GCE image"
This reverts commit ce208996bf954a87d68789bb6a9000256f016c77.

Updates golang/go#29822

Change-Id: I47e75d135e092ee9d5a80c66e91c1048c67cd5f6
Reviewed-on: https://go-review.googlesource.com/c/158542
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index 36d0853..b0e0985 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -256,7 +256,7 @@
 		OwnerGithub: "0intro",
 	},
 	"host-plan9-386-gce": &HostConfig{
-		VMImage:            "plan9-386-v8",
+		VMImage:            "plan9-386-v7",
 		Notes:              "Plan 9 from 0intro; GCE VM is built from script in build/env/plan9-386",
 		buildletURLTmpl:    "http://storage.googleapis.com/$BUCKET/buildlet.plan9-386",
 		goBootstrapURLTmpl: "https://storage.googleapis.com/$BUCKET/gobootstrap-plan9-386.tar.gz",
@@ -1468,6 +1468,14 @@
 		HostType:       "host-plan9-386-gce",
 		numTestHelpers: 1,
 		MaxAtOnce:      2,
+		ShouldRunDistTest: func(distTestName string, isTry bool) bool {
+			switch distTestName {
+			case "api",
+				"go_test:cmd/go": // takes over 20 minutes without working SMP
+				return false
+			}
+			return true
+		},
 	})
 	addBuilder(BuildConfig{
 		Name:              "windows-amd64-2008",
diff --git a/env/plan9-386/make.bash b/env/plan9-386/make.bash
index 065b4c0..4cf52e7 100755
--- a/env/plan9-386/make.bash
+++ b/env/plan9-386/make.bash
@@ -142,12 +142,6 @@
 send "mv /tmp/plan9.ini /n/9fat/plan9.ini\n"
 
 expect -exact "term% "
-send "sed /^\\\*nomp'='/d /n/9fat/plan9.ini >/tmp/plan9.ini\n"
-
-expect -exact "term% "
-send "mv /tmp/plan9.ini /n/9fat/plan9.ini\n"
-
-expect -exact "term% "
 send "sed s/9pcf/9pccpuf/ /n/9fat/plan9.ini >/tmp/plan9.ini\n"
 
 expect -exact "term% "