Cleanup SB pipelines

[MichaelSimons]

The SB and SB-Lite pipelines should be cleaned-up from main after the Microsoft build ships from the UB.

The SB pipeline will need to be retained for servicing branches but the SB-Lite pipeline can be removed entirely as it only runs on main.

[mthalman]

The SB pipeline is currently the only means we have to do PR validation for SB. So the removal of it is dependent on #5061 which will enable PR validation for SB in the UB pipeline.

[MichaelSimons]

@mthalman, what work remains here? TIA.

[mthalman]

Fixed by dotnet/dotnet#549