Remove repo level source build CI/PR legs

[MichaelSimons]

Per the UB Validation Strategy, the repo level source build CI/PR legs should be removed. The basis for comes from the difficulty of validating source build, specifically correctly detecting prebuilts, at the repo level. Validating source build is best done in the VMR. Support will be added for optional VMR based source build legs in repo PR validation.

This epic tracts the removal of repo level source build CI/PR legs.

[ViktorHofer]

For all these we should also remove conditions that apply only to the repo source-build: DotNetBuildSourceOnly==true && DotNetBuildOrchestrator!=true

[MichaelSimons]

In discussing this, there is value is leaving the SB PR/CI legs as a way to validate the SB infrastructure at the repo level. These legs should be considered optional. If a repo opts into enabling a VMR SB validation leg, then the repo level SB leg should be disabled as it would add no additional value. If a repo has no special source only conditions then it may make sense to disable the source build leg.

Closing as we will not being doing a mass removal.