Remove direct kernel metric removal patterns

## Goal

Replace all direct kernel metric removal with sliding window observer.

## Tasks

1. Remove direct removal in `collect_container_stat()` (stats.py:658-661)
1. Remove or refactor `remove_kernel_metric()` calls from agent.py:
- Line 1505 in `_handle_clean_event()`
- Line 1641 in `purge_kernel()`
1. Ensure sliding window observer handles all cleanup cases

## Dependencies

Requires sliding window observer implementation (BA-3521 subtask 2).

JIRA Issue: BA-3524