Simplify cross-repo/default labels

[cassidyjames]

We can define the default issue/PR labels for new repos at the GitHub org level, but I don't think these apply retroactively to existing repos. Regardless, we should clean up the defaults and the ones in use on repos to solve a few things:

• The defaults predate the "Issue Type" drop-down, meaning they duplicate it in some cases (like bug → Type: Bug, enhancement → Type: Feature, at least)
• Same with duplicate which is better handled by built-in GitHub features (you can close an issue explicitly as a duplicate of another)