revert: restore #![allow(dead_code)] - CI clippy -D warnings conflict (#3979)

revert: restore #![allow(dead_code)] - clippy -D warnings treats warn as error

The #742 PR changed #![allow(dead_code)] to #![warn(dead_code)], but
CI runs clippy with -D warnings which turns warnings into errors.
This caused CI failures across multiple PRs.

Reverting to #![allow(dead_code)] until the dead code is actually
cleaned up. The 189 warnings in ecstore should be fixed incrementally
by deleting dead code and adding item-level allows, not by changing
the crate-level policy.

crates/obs/src/metrics/schema/node_disk.rs

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#![warn(dead_code)] // Gradually clean up; see https://github.com/rustfs/backlog/issues/742
+#![allow(dead_code)]
 
 use crate::{MetricDescriptor, MetricName, MetricSubsystem, new_gauge_md};
 use std::sync::LazyLock;