Removing unused pubsubSecret from sources.spec

[nachocano]

Fixes #548

Proposed Changes

• 🧽 Remove pubSubSecret from the spec of current Sources. This was needed when we had the job-based reconcilers. We do not need it anymore, as we can just use spec.secret for the data plane.
• 🧽 Added some validations and few other cleanups.

Release Note

The sources do not have a `spec.pubSubSecret` field anymore. If you were setting values there, you can just use `spec.secret` instead.

[nachocano]

/assign @capri-xiyue

[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/apis/duck/v1alpha1/pubsub_types.go	6.2%	6.7%	0.4
pkg/apis/events/v1alpha1/cloudauditlogssource_validation.go	83.3%	81.0%	-2.4
pkg/apis/events/v1alpha1/cloudpubsubsource_validation.go	88.9%	90.6%	1.7
pkg/apis/events/v1alpha1/cloudschedulersource_validation.go	93.1%	91.7%	-1.4
pkg/apis/events/v1alpha1/cloudstoragesource_validation.go	93.5%	92.3%	-1.2
pkg/apis/pubsub/v1alpha1/pullsubscription_validation.go	87.5%	88.4%	0.9
pkg/reconciler/pubsub/resources/pullsubscription.go	85.7%	100.0%	14.3

[knative-prow-robot]

@nachocano: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-google-knative-gcp-go-coverage	a3830db	link	/test pull-google-knative-gcp-go-coverage

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[capri-xiyue]

/lgtm

[capri-xiyue]

/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: capri-xiyue, nachocano

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