Fanout trying to create handler goroutine for knative eventing broker/trigger

[zhongduo]

Describe the bug
When the cluster has knative eventing broker and trigger, the broker fanout pod will try
to create handler goroutine for the knative eventing trigger, which will result in the following error
in stackdriver log.

{
insertId: "d224dig4e75pew"
jsonPayload: {
logger: "broker-fanout"
message: "handler for broker has stopped with error"
commit: "5e6049b"
knative.dev/pod: "default-brokercell-fanout-7cc748475c-f69cz"
caller: "handler/fanout.go:180"
logging.googleapis.com/sourceLocation: {3}
timestamp: "2021-01-26T15:11:24.507992023Z"
broker: "BROKER:event-example//default"
error: "rpc error: code = NotFound desc = Resource not found (resource=cre-bkr_event-example_default_3ac6629b-e9aa-4f78-b2e6-4233102b2a99)."
stacktrace: "github.com/google/knative-gcp/pkg/broker/handler.(*FanoutPool).SyncOnce.func2.1
	github.com/google/knative-gcp/pkg/broker/handler/fanout.go:180
github.com/google/knative-gcp/pkg/broker/handler.(*Handler).Start.func1
	github.com/google/knative-gcp/pkg/broker/handler/handler.go:76"
}
resource: {2}
timestamp: "2021-01-26T15:11:24.508537436Z"
severity: "ERROR"
labels: {5}
logName: "projects/jimmy-knative-dev/logs/stderr"
receiveTimestamp: "2021-01-26T15:11:25.014272940Z"
}

Expected behavior
Fanout will not try to create the handler for knative eventing triggers.

To Reproduce
After installing both knative eventing and knative gcp, follow https://knative.dev/docs/eventing/getting-started/ to create eventing broker and trigger. Then check the log in stackdriver.