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/collectors/mod.rs

@@ -1,13 +1,6 @@
 use async_trait::async_trait;
 use cm_dashboard_shared::{Metric, StatusTracker};
-use std::time::Duration;
 
-/// Performance metrics for a collector
-#[derive(Debug, Clone)]
-pub struct PerformanceMetrics {
-    pub last_collection_time: Duration,
-    pub collection_efficiency_percent: f32,
-}
 
 pub mod backup;
 pub mod cpu;
@@ -28,10 +21,6 @@ pub trait Collector: Send + Sync {
     /// Collect all metrics this collector provides
     async fn collect(&self, status_tracker: &mut StatusTracker) -> Result<Vec<Metric>, CollectorError>;
 
-    /// Get performance metrics for monitoring collector efficiency
-    fn get_performance_metrics(&self) -> Option<PerformanceMetrics> {
-        None
-    }
 
 }