After deleting a task associated events aren't removed from topic event list

Steps to reproduce:
1. Create any task with alert node and specified topic name
2. Send data for generating event from the alert node
3. Delete the task via HTTP API
4. Check output at /kapacitor/v1/alerts/topics//events

Checked on official docker images https://hub.docker.com/_/kapacitor/
1.3.3-alpine
1.4.1-alpine

Note: The issue isn't very annoying however looks like the problem causes another issues with alert topic in case of active update the tasks configuration via API. But I haven't found steps to reproducing for other, probably correlated, issues yet.

Start by reproducing the task deletion flow with an alert node and topic, then query the HTTP API endpoint /kapacitor/v1/alerts/topics//events. Compare the event list before and after deleting the task; done means events associated with the deleted task no longer appear. The report was checked against the 1.3.3-alpine and 1.4.1-alpine Docker images.

Written by the indexing model from the issue text.