Do not label auto-generated toolchain PR

Creating the PR with the Z-BenchCI label will not actually trigger the
performance benchmarking workflow: that workflow is conditional on the
label newly being attached to an existing PR. Consequently, one had to
remove and then re-add the label for the workflow to be triggered (see
 model-checking#2752 for an example).

.github/workflows/toolchain-upgrade.yml

@@ -74,7 +74,6 @@ jobs:
           branch: toolchain-${{ env.next_toolchain_date }}
           delete-branch: true
           title: 'Automatic toolchain upgrade to nightly-${{ env.next_toolchain_date }}'
-          labels: Z-BenchCI
           body: >
             Update Rust toolchain from nightly-${{ env.current_toolchain_date }} to
             nightly-${{ env.next_toolchain_date }} without any other source changes.