Make labels a bit easier for build cops 👮‍♀️

I was build cop today and I noticed in our "build cop runbook" we
talked about the purposes of a few labels, but definitely not all of
them. Since this is the authoritative source on what labels we have, I
think it makes sense to make this our documentation as well, and also to
clean out labels we don't need (many are copied from knative which
copied from kubernetes)

Removed some labels:
- triage/duplicate: Can use "Duplicate of" instead
  (https://help.github.com/en/github/managing-your-work-on-github/about-duplicate-issues-and-pull-requests)
- triage/needs-information: Could use zenhub "waiting for response"
  and/or "kind/question" instead
- triage/not-reproducible, triage/support, triage/unresolved - not
  really sure what these would mean for us
- kind/api-change, "api-review": Can use "area/api" instead
- area/release, area/dogfooding, area/test-infra - We have a separate
  plumbing repo where we can put these issues instead
- kind/failing-test, kind/flake - Seems oddly specific. Instead IMO this is kind of
  bug, and in fact something we should drop everything to fix.
- lifecycle/* - We do not use this plugin
- needs-kind, needs-sig, release-note*, shrug - We don't use these

Remove labels from repos that aren't used:
- cli: user-study
- community: area/cdf-summit
- pipeline: maybe-next-milestone (can use priority instead), user-study,
  also old silly labels like "meaty-juicy-coding-work", okr (not using
  anymore)

Removed labels from repos that were also global