Remove all unused code and fix build warnings

- Remove unused struct fields: tier, config_name, last_collection_time
- Remove unused structs: PerformanceMetrics, PerfMonitor
- Remove unused methods: get_performance_metrics, get_collector_names, get_stats
- Remove unused utility functions and system helpers
- Remove unused config fields from CPU and Memory collectors
- Keep config fields that are actually used (DiskCollector, etc.)
- Remove unused proxy_pass_url variable and assignments
- Fix duplicate hostname variable declaration
- Achieve zero build warnings without functionality changes

agent/src/communication/mod.rs

@@ -9,7 +9,6 @@ use crate::config::ZmqConfig;
 pub struct ZmqHandler {
     publisher: Socket,
     command_receiver: Socket,
-    config: ZmqConfig,
 }
 
 impl ZmqHandler {
@@ -41,7 +40,6 @@ impl ZmqHandler {
         Ok(Self {
             publisher,
             command_receiver,
-            config: config.clone(),
         })
     }