UX: Strip multiline comments in github oneboxes (#30040)

We were already stripping comments from GitHub issue/PR oneboxes, but the regex was not correctly matching multiline comments.

lib/onebox/engine/github_pull_request_onebox.rb

@@ -12,8 +12,6 @@ module Onebox
       include Onebox::Mixins::GithubBody
       include Onebox::Mixins::GithubAuthHeader
 
-      GITHUB_COMMENT_REGEX = /(<!--.*?-->\r\n)/
-
       matches_regexp(%r{^https?://(?:www\.)?(?:(?:\w)+\.)?(github)\.com(?:/)?(?:.)*/pull})
       always_https