Avoid checked-in nuspec files

[ViktorHofer]

Checked-in nuspec files are problematic because they don't work well with different build configurations (i.e. source-build vs non-source-build). They also don't automatically get new NuGet features like a package readme or other examples.

The checked-in efcore.nuspec file is the reason for this build break with a newer SDK: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1069086&view=logs&jobId=9050e078-31bf-5111-d8ec-8b6fa95caf9c&j=9050e078-31bf-5111-d8ec-8b6fa95caf9c&t=4df1dbeb-294c-5d6e-97b8-581fe506c270

D:\a_work\1\s.dotnet\sdk\10.0.100-preview.6.25315.102\Sdks\NuGet.Build.Tasks.Pack\build\NuGet.Build.Tasks.Pack.targets(221,5): error NU5000: Nuspec file contains a package type that is missing the name attribute. [D:\a_work\1\s\src\efcore\src\dotnet-ef\dotnet-ef.csproj]

[ViktorHofer]

@AndriySvyryd @cincuranet do you remember why these nuspecs are checked-in?

[cincuranet]

No.

[AndriySvyryd]

For the first 3 we need to include the output of a different project (ef.csproj)
For Analyzers I think that at the time the was no other way of correctly packaging it.