revert change to get PingSource specs from ConfigMaps

[lionelvillard]

Proposed Changes

• Partially reverting Add persistent cache store backed by ConfigMap. Refactor mtping #3451 causing a regression on the max number PingSources the adapter can handle.

Release Note

Docs

This PR needs to be backported to 17.

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lionelvillard

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• OWNERS [lionelvillard]

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[knative-metrics-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/adapter/mtping/adapter.go	60.9%	62.5%	1.6
pkg/adapter/mtping/controller.go	Do not exist	100.0%
pkg/adapter/mtping/pingsource.go	Do not exist	72.2%
pkg/reconciler/pingsource/controller.go	92.3%	91.7%	-0.6

[cr22rc]

/lgtm