[release/9.0-staging] Remove unneeded DiagnosticSource content

[github-actions]

Backport of #112116 to release/9.0-staging

/cc @tarekgh @noahfalk

Customer Impact

• Customer reported
• Found internally

Users who create a NetFX project and reference the System.Diagnostics.DiagnosticSource 9.0 package will encounter a build issue similar to the following:

The issue arises because the library package includes ILLink.Descriptors.LibraryBuild.xml, which is unintentionally marked to be copied during the build, even though it is not necessary. This can also cause confusion for other .NET users, as the file ILLink.Descriptors.LibraryBuild.xml is automatically created for C# projects that do not use an SDK-style format, impacting many older codebases. More details can be found in the related issue: #112110

Regression

• Yes
• No

This was unintentional change done as part of the PR #106014

Testing

This has been tested with .NET 10 branch and confirmed fixing the reported issue.

Risk

Very low as we are not changing any code more than updating the csproj to not copy the file ILLink.Descriptors.LibraryBuild.xml during the build.

[High/Medium/Low. Justify the indication by mentioning how risks were measured and addressed.]

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

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[tarekgh]

CC @tommcdon @ericstj @artl93 @jkotas

[noahfalk]

LGTM

@tarekgh - the issue notes say that testing confirmed this worked. I didn't test it before making the .NET 10 fix so just want to make sure there was no confusion there. If you tested it then all is well :)

[tarekgh]

the issue notes say that testing confirmed this worked. I didn't test it before making the .NET 10 fix so just want to make sure there was no confusion there. If you tested it then all is well :)

Yes, I confirmed the .NET 10 package does not contain the file ILLink.Descriptors.LibraryBuild.xml anymore (not even has the contentFiles folder). While .NET 9.0 Package has it: