github/workflows: remove contrib workflow

This job is already running in the Prow infrastructure.

Signed-off-by: Ivan Valdes <ivan@vald.es>

.github/workflows/contrib.yaml

@@ -1,18 +0,0 @@
----
-name: Test contrib/mixin
-on: [push, pull_request]
-permissions: read-all
-jobs:
-  test:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
-      - id: goversion
-        run: echo "goversion=$(cat .go-version)" >> "$GITHUB_OUTPUT"
-      - uses: actions/setup-go@f111f3307d8850f501ac008e886eec1fd1932a34 # v5.3.0
-        with:
-          go-version: ${{ steps.goversion.outputs.goversion }}
-      - run: |
-          set -euo pipefail
-
-          make -C contrib/mixin tools test