Delete existing release asset before releasing

Use mknejp/delete-release-assets to delete existing asset.
This approach might not be optimal if softprops/action-gh-release
supports upserting [1].

[1] softprops/action-gh-release#134

.github/workflows/generate_pdf.yml

@@ -15,6 +15,13 @@ jobs:
       - uses: actions/checkout@v2
       - name: Build
         run: make all
+      - name: Delete old release asset
+        uses: mknejp/delete-release-assets@v1
+        with:
+          token: ${{ github.token }}
+          fail-if-no-assets: false
+          tag: latest
+          assets: lkmpg.pdf
       - name: Release
         uses: softprops/action-gh-release@v1
         with: