Update GHE commit message template

Removed Reviewers/subscribers from the sync commit messages, they're
hangovers from phab diffs and don't mean anything in a GitHub world.

theme-utils.mjs

@@ -1019,10 +1019,6 @@ Summary:
 ${logs}
 
 Test Plan: Execute Smoke Test
-
-Reviewers:
-
-Subscribers:
 `;
 }