Clean up dead code in illink targets (#85741)

Removes TFM version checks, and adds a warning when using
explicit ILLink package reference.

We reference ILLink as a packagereference that versions with the
runtime, so the 8.0 linker is only supported when targeting
`net8.0`, and we can remove the TFM checks. We don't want to
support someone adding a PackageReference to an 8.0 linker, while
targeting an older TFM, in an attempt to get newer linker bits
but with settings that were closer to what we shipped with that
TFM.

---------

Co-authored-by: Marek Safar <[email protected]>
Co-authored-by: Vitek Karas <[email protected]>

Author: 3 people

src/coreclr/nativeaot/BuildIntegration/Microsoft.DotNet.ILCompiler.SingleEntry.targets

@@ -43,7 +43,7 @@
   <Target Name="GenerateILCompilerExplicitPackageReferenceWarning" Condition="'$(SuppressGenerateILCompilerExplicitPackageReferenceWarning)' == '' and '$(ILCompilerTargetsPath)' != ''"
       BeforeTargets="ImportRuntimeIlcPackageTarget">
     <Warning Condition="'%(PackageReference.Identity)' == 'Microsoft.DotNet.ILCompiler' And '%(PackageReference.IsImplicitlyDefined)' != 'true'"
-             Text="Delete explicit 'Microsoft.DotNet.ILCompiler' package reference in your project file. Explicit 'Microsoft.DotNet.ILCompiler' package reference can run into version errors." />
+             Text="Delete explicit 'Microsoft.DotNet.ILCompiler' package reference in your project file to avoid versioning problems." />
   </Target>
 
   <!-- Locate the runtime package according to the current target runtime -->

src/tools/illink/src/ILLink.Tasks/ILLink.Tasks.csproj

@@ -28,7 +28,6 @@
     <!-- Note: 'build/Microsoft.NET.ILLink.targets' should not match the package name, because we don't want the targets
          to be imported by nuget. The SDK will import them in the right order. -->
     <None Include="**\*.props;**\*.targets" Pack="true" PackagePath="%(RecursiveDir)%(Filename)%(Extension)" />
-    <None Include="build/6.0_suppressions.xml" Pack="true" PackagePath="build/" />
     <None Include="..\ILLink.RoslynAnalyzer\Microsoft.NET.ILLink.Analyzers.props" Pack="true" PackagePath="build/" />
   </ItemGroup>