docs(io-metrics): remove stale unified-config docs left by #6008 (#6039)

PR #6008 deleted crates/io-metrics/src/config.rs but the crate docs still taught the deleted API: both READMEs kept the Unified Configuration sections, module-tree entries and ./src/config.rs links, the example kept an orphaned numbered comment, and the io-core/io-metrics changelog had no removal record. Scrub all of it; keep cache_config.rs references, which are still real.

crates/io-metrics/examples/metrics_example.rs

@@ -29,9 +29,7 @@ fn main() {
     // 3. Access tracking example
     access_tracker_example();
 
-    // 4. Unified configuration example
-
-    // 5. Metrics recording example
+    // 4. Metrics recording example
     metrics_recording_example();
 }