Auto remove the `do-not-merge/hold` for cherry-pick PR when the conflicts are resolved

[wuhuizuo]

What

Make the bot can auto remove the hold label added by itself when the conflicts are resolved for the PR.

Why

Now the bot will add the label when conflicts are detected in the cherry-pick PR, but assignees always forgot to remove the label by GitHub UI or comments after resolving the conflicts.

How

1. Only remove it when it was added by the bot (ti-chi-bot or ti-chi-bot[bot]).
2. Give the tips in comments before adding/removing the label.