Fix session deletion not updating the sessions view immediately

When deleting a local chat session, LocalAgentsSessionsController fired
a removal event but did not remove items from its internal _items map.
This caused AgentSessionsModel to still see the deleted session when
re-reading controller.items. The fix removes items from _items before
firing the event, consistent with AgentHostSessionListController.

Agent-Logs-Url: https://github.com/microsoft/vscode/sessions/4f110187-3767-4b48-a8bf-faafc14b3ce6

Co-authored-by: bpasero <[email protected]>