Remove overlay filesystem functionality and add detection/removal capability

- Remove unused overlay filesystem configuration and references
- Remove overlayfs sections from all profile JSON files
- Remove OVERLAY_ENABLED/OVERLAY_PROTECT_CONFIGS from config
- Update documentation to focus on tmpfs optimization
- Add overlay detection and removal functionality for cleanup
- Add remove_overlays() function with safe unmounting
- Add overlay status reporting in final optimization summary
- Add test-overlay-detection.sh for testing detection logic
- Simplify codebase by removing complex unused features
- Focus on proven desktop optimizations (tmpfs, zram, kernel)

profiles/gaming.json

@@ -48,9 +48,6 @@
       "net.core.netdev_max_backlog": 10000,
       "net.core.rmem_max": 33554432,
       "net.core.wmem_max": 33554432
-    },
-    "overlayfs": {
-      "enabled": false
     }
   },
   "gaming_specific": {