[Bug] Remove appKey in github-checks-reporter (opensearch-project#603)

github-checks-reporter is a task wrapper that provides expressive
CI feedback via GitHub checks. Currently, we don’t have our own
appKey. To avoid any confusions, this PR removed the old key.

Partically Resolved:
opensearch-project#592

Signed-off-by: Anan Zhuang <[email protected]>

Author: ananzh

github_checks_reporter.json

@@ -1,6 +1,6 @@
 {
   "appId": 26774,
   "envVars": {
-    "appKey": "KIBANA_CI_REPORTER_KEY"
+    "appKey": ""
   }
 }