Fix WebSocket log subscription management

The `saiw_ws_json_tx_browser()` function did not properly manage
WebSocket connections for task-specific log streaming. When a browser
navigated to a new task's log page, it would subscribe to the new
task's logs without unsubscribing from the previous one. This led to
the browser receiving logs from all previously viewed tasks, causing
an "avalanche of logs".

This change fixes the issue by ensuring that when a browser subscribes
to a new task's logs, it is first unsubscribed from any previous
task's logs. This is done by checking if the connection is already in
a subscription list and removing it before adding the new subscription.