Remove spec.googleServiceAccount in v1alpha1

[grac3gao-zz]

Fixes #1205

Proposed Changes

• Removing spec.googleServiceAccount, together, deleting any controller logic related on googleServiceAccount

Release Note

`spec.googleServiceAccount` has been removed. All automatic Workload Identity related controller work is now handled via `config-gcp-auth`.

[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/credentials.go	79.2%	100.0%	20.8
pkg/reconciler/events/auditlogs/auditlogs.go	89.6%	87.0%	-2.6
pkg/reconciler/events/build/build.go	78.6%	57.1%	-21.4
pkg/reconciler/events/pubsub/pubsub.go	78.6%	57.1%	-21.4
pkg/reconciler/events/scheduler/scheduler.go	81.4%	78.6%	-2.9
pkg/reconciler/events/storage/storage.go	74.7%	72.5%	-2.2
pkg/reconciler/identity/reconciler.go	65.2%	65.9%	0.7
pkg/reconciler/intevents/pullsubscription/resources/receive_adapter.go	85.3%	87.1%	1.8
pkg/reconciler/intevents/topic/resources/publisher.go	81.2%	84.6%	3.4
pkg/reconciler/messaging/channel/channel.go	79.9%	78.0%	-1.9

[nachocano]

/approve
/lgtm

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: grac3gao, nachocano

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[Harwayne]

/hold

Please add a release notes section saying that the field has been removed. The release-notes section is what we use to create the release notes and this is definitely a user-visible change.

[grac3gao-zz]

@Harwayne Done. Sorry, I forget to add release note....