Remove Automerge job (#270)

Since we don't have as many submodules as we used to, and we don't
update it as frequently as we used to, handling Dependabot PRs manually
is not as much of a chore as it used to be. Consequently, we don't need
Automerge anymore, and since we don't need the secret, we can change our
trigger back to `pull_request`.

- Remove automerge job
- Change test trigger from `pull_request_target` back to `pull_request`

.github/workflows/test_pr.yml

@@ -4,7 +4,7 @@ on:
   push:
     branches:
       - main
-  pull_request_target:
+  pull_request:
     branches:
       - main
 
@@ -61,29 +61,3 @@ jobs:
       - name: Test
         working-directory: ${{github.workspace}}/build
         run: ctest -C ${{env.BUILD_TYPE}} -L acquire-driver-zarr --output-on-failure
-
-  merge:
-    name: Automerge
-    runs-on: "ubuntu-latest"
-    needs: test
-    if: ${{ github.actor == 'dependabot[bot]' }}
-    steps:
-      - name: Checkout PR
-        uses: actions/checkout@v2
-        with:
-          ref: ${{ github.event.pull_request.head.sha }}
-          repository: ${{ github.event.pull_request.head.repo.full_name }}
-
-      - name: Approve PR
-        run: gh pr review --approve "$PR_URL"
-        env:
-          PR_URL: ${{ github.event.pull_request.html_url }}
-          GH_TOKEN: ${{ secrets.PAT }}
-
-      # Don't auto-merge major version updates
-      - name: Merge PR
-        if: ${{ steps.dependabot-metadata.outputs.update-type != 'version-update:semver-major' }}
-        run: gh pr merge --auto --squash "$PR_URL"
-        env:
-          PR_URL: ${{ github.event.pull_request.html_url }}
-          GH_TOKEN: ${{ secrets.PAT }}