[Actions] Revert addition of auto-labeller action for bottle :unneeded (#48929)

- I forgot that GitHub doesn't support secrets in Actions from forked repos, [until there was a legitimate use case for adding a label](https://github.com/Homebrew/homebrew-core/pull/48927), so the action will fail:

```
/opt/hostedtoolcache/Ruby/2.6.3/x64/lib/ruby/gems/2.6.0/gems/octokit-4.15.0/lib/octokit/response/raise_error.rb:16:in `on_complete': POST https://api.github.com/repos/Homebrew/homebrew-core/issues/48927/labels: 403 - Resource not accessible by integration // See: https://developer.github.com/v3/issues/labels/#add-labels-to-an-issue (Octokit::Forbidden)
```

- Given our PR workflow is "forked repo for everything", we can't do this until
  GitHub supports using secrets in forked PRs. It's a [requested
  feature](https://github.community/t5/GitHub-Actions/how-to-use-GITHUB-TOKEN-for-PRs-from-forks/td-p/37450).
- Maybe at some other point in the future, we can revisit this and other
  auto-labelling actions.
- I enjoyed making it nonetheless, and now I have "forked repos" to add to my
  testing for future Actions proof-of-concepts!

----

Reverts #48911 and supersedes #48919.

.github/workflows/labels.yml

@@ -1,19 +0,0 @@
-name: Label PRs according to criteria
-
-on:
-  pull_request:
-    paths:
-    - 'Formula/*.rb'
-
-jobs:
-  label:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-      - uses: actions/setup-ruby@v1
-        with:
-          ruby-version: '2.6.3'
-      - run: gem install octokit
-      - run: ruby .github/workflows/scripts/add-labels.rb
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/scripts/add-labels.rb

@@ -1,27 +0,0 @@
-require "octokit"
-
-client = Octokit::Client.new(access_token: ENV["GITHUB_TOKEN"])
-
-repo = ENV["GITHUB_REPOSITORY"]
-pr_number = ENV["GITHUB_REF"].split("/")[2]
-
-pr_files = client.pull_request_files(repo, pr_number)
-
-pr_labels = {
-  no_bottles: 0
-}
-
-pr_files.each do |file|
-  formula = file[:filename]
-
-  if File.exist?(formula) && File.read(formula).match(/bottle :unneeded/)
-    pr_labels[:no_bottles] += 1
-  end
-end
-
-if pr_files.count == pr_labels[:no_bottles]
-  puts "Adding the 'no-bottles' label."
-  client.add_labels_to_an_issue(repo, pr_number, ["no-bottles"])
-else
-  puts "Not labelling this PR as there's a mixture of bottled and unbottled formulae."
-end