Delete the main-ub branch

[MichaelSimons]

Once the binary cloaks have been removed from the main branch (see #4087). The main-ub branch as well as the backing infrastructure to sync it should be removed/cleaned up.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[ViktorHofer]

As part of this work, pipeline triggers in AzDO dnceng-public/public and dnceng/internal need to be updated. Otherwise unified build pipelines would stop running. In the pipeline settings, the default branch needs to be updated as well.