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
35 jobs for !6781 with vs-csproj-scripts in 28 minutes (queued for 6 seconds)
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