internal/migration: stop running plan9/{386,arm} builders

These builders have been migrated to LUCI.

For golang/go#67761.

Change-Id: I5a1672f42db2f873c94e87aa14f18ec85de92cf5
Reviewed-on: https://go-review.googlesource.com/c/build/+/607656
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

internal/migration/migration.go

@@ -84,6 +84,9 @@ var BuildersPortedToLUCI = map[string]bool{
 
 	"solaris-amd64-oraclerel": true, // Available as https://ci.chromium.org/p/golang/builders/ci/gotip-solaris-amd64.
 
+	"plan9-386": true, "plan9-386-0intro": true, // Available as https://ci.chromium.org/p/golang/builders/ci/gotip-plan9-386.
+	"plan9-arm": true, // Available as https://ci.chromium.org/p/golang/builders/ci/gotip-plan9-arm.
+
 	// WebAssembly builders.
 	"js-wasm-node18":       true, // Available as https://ci.chromium.org/p/golang/builders/ci/gotip-js-wasm.
 	"wasip1-wasm-wasmedge": true, // Would be 'wasip1-wasm_wasmedge' but put off until go.dev/issue/60097 picks up activity.