Ensure we can run the not-supported source generation in source build

[ericstj]

Follow up issue for #33631

We should remove this file as it should no longer be needed: https://github.com/dotnet/runtime/blob/master/eng/notSupported.SourceBuild.targets

We should also change the condition on

runtime/src/libraries/Directory.Build.targets

Line 139 in f0461cb

<PackageReference Include="Microsoft.DotNet.GenAPI" Condition="'$(DotNetBuildOffline)' != 'true'" Version="$(MicrosoftDotNetGenApiVersion)" PrivateAssets="all" IsImplicitlyDefined="true" />

to

    <PackageReference Include="Microsoft.DotNet.GenAPI" Condition="'$(DotNetBuildFromSource)' != 'true'" Version="$(MicrosoftDotNetGenApiVersion)" PrivateAssets="all" IsImplicitlyDefined="true" />

While doing this, review the original changes that added this behavior to ensure there are no other places that need to be cleaned up.

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

Tagging subscribers to this area: @safern, @ViktorHofer
See info in area-owners.md if you want to be subscribed.