Remove roslyn source build CI/PR legs

[MichaelSimons]

See #5006 for background.

Remove the following:

1. Source build CI/PR legs
2. Source build prebuilts baselines file (eng/SourceBuildPrebuiltBaseline.xml)

[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 dotnet/arcade-services#4539, 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.