remove event emit for normal trigger reconcile

[vaikas]

Addresses #knative/pkg#1520

While Matt and I were debugging things related to chaos, and in particular what happens to 'nop' controllers, we started looking at places were we are doing unnecessary api server calls. This is removing one of them.
Overarching issue tracking in /pkg
knative/pkg#1520

Proposed Changes

• Do no emit trigger reconciled successfully event

Release Note

- 🧽 Update or clean up current behavior
When Trigger is reconciled, do not emit an event for it

Docs

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: vaikas

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [vaikas]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[knative-metrics-robot]

The following is the coverage report on the affected files.
Say /test pull-knative-eventing-go-coverage to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/reconciler/mtbroker/broker.go	86.5%	86.5%	-0.1

[mattmoor]

/lgtm