Make official build assetless + remove source-build

[MiYanni]

Follow-up to: #9555
Follow-up to: #9722

Summary

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This PR just adds this block for running an assetless build only in the official pipeline:

- template: /eng/common/templates-official/jobs/jobs.yml@self
  parameters:
    enablePublishBuildArtifacts: true
    enablePublishBuildAssets: false
    enablePublishTestResults: false
    publishAssetsImmediately: true
    isAssetlessBuild: true
    enableTelemetry: true

The only other change was disabling the source-build job by removing these parameters from both the PR yml and the internal yml:

enableSourceBuild: true
sourceBuildParameters:
  enableInternalSources: true

Builds

• Official: https://dnceng.visualstudio.com/internal/_build/results?buildId=2880345&view=results
• Unofficial: https://dnceng.visualstudio.com/internal/_build/results?buildId=2880346&view=results