Drop Codecov.Tool PackageDownload — coverage upload to codecov.io is unused

[ChrisonSimtian]

build/_build.csproj declares <PackageDownload Include="Codecov.Tool" Version="[1.13.0]" />. The release workflow doesn't push to codecov, and IReportCoverage.ReportToCodecov is false in Build.cs. Removing the download saves a build step and one less .NET tool on the runner.

If we ever want codecov, it's a 1-line add back.

Refs the dependency audit. Done when:

• PackageDownload removed from build/_build.csproj
• Any related guards in Build.cs / coverage targets cleaned up

[ChrisonSimtian]

Closed in 97ea857 — Codecov.Tool PackageDownload removed from build/_build.csproj. Build.cs's IReportCoverage.ReportToCodecov => false kept as the explicit toggle for clarity.