Add support for removing branches in update-rules

[akhilerm]

The update-rules command is used to add new branches to the publishing-bot rules. Ref: https://github.com/kubernetes/publishing-bot/tree/master/cmd/update-rules

We also have a use-case of removing rules of EOL branches similar to kubernetes/kubernetes@43c841f.

The update-rules command can be modified to have a new flag , which when provided will remove the corresponding branch from the rules file.

[mohitsharma-in]

@akhilerm Can i pick this up ?

[akhilerm]

sure. Thank you.
/assign @mohitsharma-in