Revert incorrect emergency message (#1275)

Sleep-deprived me set the wrong field :(
diff --git a/buildkite/emergency.yml b/buildkite/emergency.yml
index 64d4efd..3207eea 100644
--- a/buildkite/emergency.yml
+++ b/buildkite/emergency.yml
@@ -18,5 +18,5 @@
 # Instead, simply replace all values with empty strings ("").
 ---
 message: ""
-issue_url: "MacOS machines are offline due to network maintenance. We expect the outage to continue until 6pm EDT today."
+issue_url: ""
 last_good_bazel: ""