Delete diff artifact after use in "Manage PRs" workflow

A workflow artifact is used to transfer the PR diff file from the `diff` job to the `parse` job. Once the artifact has
been downloaded by the `parse` job, it no longer serves any purpose.

It's possible the artifact might serve as a vector for exporting secrets from the workflow. Even though I don't have any
specific reasons to believe it is possible to cause secrets to be written to the artifact and the repository doesn't
currently have any secrets beyond `GITHUB_TOKEN`, nor need for any, it's still best to remove the unnecessary artifact.

.github/workflows/manage-prs.yml

@@ -109,6 +109,11 @@ jobs:
           path: ${{ needs.diff.outputs.path }}
           name: ${{ needs.diff.outputs.artifact }}
 
+      - name: Remove no longer needed artifact
+        uses: geekyeggo/delete-artifact@v1
+        with:
+          name: ${{ needs.diff.outputs.artifact }}
+
       - name: Parse request
         id: parse-request
         run: |