Remove buildkite from portdb CI tests

.github/workflows/tests.yml

@@ -278,13 +278,7 @@ jobs:
       - uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}
-      - name: Patch Buildkite-specific test scripts
-        run: |
-          sed -i -e 's/host="postgres"/host="localhost"/' .buildkite/scripts/postgres_exec.py
-          sed -i -e 's/host: postgres/host: localhost/' .buildkite/postgres-config.yaml
-          sed -i -e 's|/src/||' .buildkite/{sqlite,postgres}-config.yaml
-          sed -i -e 's/\$TOP/\$GITHUB_WORKSPACE/' .coveragerc
-      - run: .buildkite/scripts/test_synapse_port_db.sh
+      - run: ci/scripts/test_synapse_port_db.sh
 
   complement:
     if: ${{ !failure() && !cancelled() }}