Remove unnecessary DotNetBuildOffline switches

[ViktorHofer]

DotNetBuildOffline is set in two test projects in the fsharp repo. Test projects are already filtered out when building from source, so those switches are unnecessary.

We plan to overhaul the source build control switches and remove this one completely. FSharp was the only repo that was still using it.

[github-actions]

✅ No release notes required

[vzarytovskii]

@ViktorHofer these are not typical test projects, they are "normal" projects which we use to test trimming of freshly built FSharp core with fresh compiler. We use those to correctly restore packages, so some more work will be needed to remove those. I can say now what changes are required.

It will probably need some additional adjustment for nuget sources used to make sure only local directory is used.

[ViktorHofer]

so some more work will be needed to remove those

Everything under "/tests" is already excluded in a source build today:

fsharp/tests/Directory.Build.props

Line 6 in 97923c9

<ExcludeFromSourceBuild>true</ExcludeFromSourceBuild>

As the DotNetBuildOffline switch is only respected when building from source, the properties are just dead code and don't impact other kinds of builds.

[ViktorHofer]

This PR depends on #16488

[ViktorHofer]

@T-Gro / @vzarytovskii can you please merge this PR in?