root-signing: Remove status checks from branch protection

[jku]

tuf-on-ci (the new tooling for root-signing) commits the online signature without a PR existing: this means status checks are not available for that commit.

This should fix sigstore/root-signing#1349 and enables sigstore/root-signing#1320.

While the status checks here would be nice, we're not really losing that much when we remove dco and yamllint...

[github-actions]

🍹 preview on sigstore-github-sync/sigstore/github-prod

Pulumi report

  Previewing update (sigstore/github-prod)

View Live: https://app.pulumi.com/sigstore/sigstore-github-sync/github-prod/previews/4f1b1bba-7768-4ab2-bf94-16ca820010d8

@ Previewing update.....
pulumi:pulumi:Stack: (same)
  [urn=urn:pulumi:github-prod::sigstore-github-sync::pulumi:pulumi:Stack::sigstore-github-sync-github-prod]
  ~ github:index/repository:Repository: (update) 🔒
      [id=model-transparency]
      [urn=urn:pulumi:github-prod::sigstore-github-sync::github:index/repository:Repository::model-transparency]
      [provider=urn:pulumi:github-prod::sigstore-github-sync::pulumi:providers:github::default_6_2_3::dfb01c0f-bb2a-45c2-9d92-e6152b5cee06]
    ~ pages: {
        - source: {
            - branch: "main"
            - path  : "/"
          }
        - source: {
            - branch: "main"
            - path  : "/"
          }
      }
  ~ github:index/branchProtection:BranchProtection: (update)
      [id=BPR_kwDOFotDCM4DA1RL]
      [urn=urn:pulumi:github-prod::sigstore-github-sync::github:index/branchProtection:BranchProtection::root-signing-main]
      [provider=urn:pulumi:github-prod::sigstore-github-sync::pulumi:providers:github::default_6_2_3::dfb01c0f-bb2a-45c2-9d92-e6152b5cee06]
    ~ requiredStatusChecks: [
        ~ [0]: {
                ~ contexts: [
                    - [0]: "DCO"
                    - [1]: "yamllint"
                  ]
              }
      ]
@ Previewing update....
Resources:
  ~ 2 to update
  590 unchanged