Deleting EventTypes when changing sink to a non-Broker kind

[nachocano]

Proposed Changes

• Changing event type reconciler to delete EventTypes of Sources whenever they point to a non-Broker sink.
• Added UTs.

Release Note

NONE

[nachocano]

/assign @n3wscott
last one Scott, I promise :)
This is the eventing-sources hopefully last registry fix :)

[nachocano]

This should be merged before the 0.6 cut.

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
contrib/gcppubsub/pkg/reconciler/gcppubsubsource.go	90.6%	90.5%	-0.1
contrib/kafka/pkg/reconciler/kafkasource.go	89.2%	89.1%	-0.2
pkg/reconciler/githubsource/githubsource.go	85.6%	85.5%	-0.1

[vaikas]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nachocano, vaikas-google

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