VS: Revert "Add missing label in C# project-build events"
Revert commit b284a21f (VS: Add missing label in C# project-build events, 2021-09-03, v3.22.0-rc1~156^2). The change broke cases using multiple successful custom commands. Revert it pending further investigation into the interaction of the generated script code with `Microsoft.Common.CurrentVersion.targets`, and whether this is needed for all managed projects or just C# projects. Also add a test covering the case that was broken. Fixes: #22964 Issue: #21440