Delete outdated comment when a PR update removes all notify owners

[rcebulko]

Context: ampproject/amphtml#30230 (comment)
Requires:

• Add deleteComment to owners/src/api/github.js#GitHub
• Add logic within owners/src/notifier.js#createNotificationComment to delete the existing comment if the notification set is empty
• Improve logic for identifying Owners Bot comment vs. other bot comments