CI: replace DCO check with github.com/probot/dco

Signed-off-by: Akihiro Suda <[email protected]>

Author: AkihiroSuda

.github/workflows/test.yml

@@ -8,22 +8,6 @@ on:
   pull_request:
 
 jobs:
-  project:
-    name: Project Checks
-    runs-on: ubuntu-20.04
-    timeout-minutes: 20
-    steps:
-      - uses: actions/setup-go@v2
-        with:
-          go-version: 1.16.x
-      - uses: actions/checkout@v2
-        with:
-          fetch-depth: 25
-      - name: "Check DCO sign"
-        run: |
-          go get -u github.com/vbatts/git-validation
-          ./hack/validate-dco.sh
-
   golangci-lint:
     runs-on: ubuntu-20.04
     timeout-minutes: 20