Remove repo level source build CI/PR legs #5006
Description
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.