feat(backend): Remove PipelineSpec Template storage from ObjStore responsibilies. Fixes #10509

[gmfrasca]

Description of your changes:
Fixes #10509

The PipelineSpec definition is currently stored in two places, DB and ObjStore, creating the potential for a competing source-of-truth problem. The ObjStore copy doesn't appear to actually be used/retrieved anywhere, so remove it from the list of responsibilities

Checklist:

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[google-oss-prow]

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[HumairAK]

/test kubeflow-pipeline-backend-test

[google-oss-prow]

@gmfrasca: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
kubeflow-pipeline-upgrade-test	84ebbf7	link	false	/test kubeflow-pipeline-upgrade-test
kubeflow-pipeline-e2e-test	84ebbf7	link	false	/test kubeflow-pipeline-e2e-test
kubeflow-pipeline-backend-test	84ebbf7	link	true	/test kubeflow-pipeline-backend-test

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