Use generic pkg/resources helpers for endpoint counting

[vagababov]

/lint

Part of #1997

Proposed Changes

• use the helpers we already have for endpoint status consideration
• remove the redundant code
• remove the redundant tests

/cc @mattmoor @dgerd

[knative-prow-robot]

@vagababov: 0 warnings.

In response to this:

/lint

Part of #1997

Proposed Changes

• use the helpers we already have for endpoint status consideration
• remove the redundant code
• remove the redundant tests

/cc @mattmoor @dgerd

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[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/reconciler/v1alpha1/revision/reconcile_resources.go	90.3%	90.8%	0.6
pkg/reconciler/v1alpha1/serverlessservice/serverlessservice.go	Do not exist	89.1%

[mattmoor]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mattmoor, vagababov

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