[ci] Remove fast check cancel workflow (#7455)

.github/workflows/cancel_fastcheck_when_ready.yml

-name: Cancel fastcheck run when PR is marked ready
-
-on:
-  pull_request:
-    types: [labeled]
-
-jobs:
-  cancel-buildkite-run:
-    if: github.event.label.name == 'ready'
-    runs-on: ubuntu-latest
-    steps:
-      - name: Cancel fastcheck run on Buildkite
-        env:
-          BUILDKITE_TOKEN: ${{ secrets.BUILDKITE_TOKEN }}
-          BUILDKITE_ORG: vllm
-          BUILDKITE_PIPELINE: fastcheck
-        run: |
-          FULL_BRANCH=${{ github.event.pull_request.head.repo.owner.login }}:${{ github.event.pull_request.head.ref }}
-          echo "PR branch: $FULL_BRANCH"
-          LATEST_BUILD_NUMBER=$(curl -H "Authorization: Bearer $BUILDKITE_TOKEN" \
-            "https://api.buildkite.com/v2/organizations/$BUILDKITE_ORG/pipelines/$BUILDKITE_PIPELINE/builds?branch=$FULL_BRANCH&state[]=scheduled&state[]=running" \
-            | jq -r '.[0].number')
-          echo "Latest build number: $LATEST_BUILD_NUMBER"
-          curl -X POST -H "Authorization: Bearer $BUILDKITE_TOKEN" \
-            "https://api.buildkite.com/v2/organizations/$BUILDKITE_ORG/pipelines/$BUILDKITE_PIPELINE/builds/$LATEST_BUILD_NUMBER/cancel"