More cleanup in benchmarks

[psfinaki]

This:

• creates Directory.Build.props for benchmarks for some common project props as promised here
• properly propagates BDN version across benchmarks (minor alignment was necessary)
• removes TaskPerf projects because:
  • this is what we've at least agreed on
  • it's not good benchmark material (mostly uses System.Diags.Stopwatch instead of BDN)

[github-actions]

✅ No release notes required

[psfinaki]

/azp run

[azure-pipelines]

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