Seeing duplicate analyzers after running try-convert

[KSchlobohm]

If the analyzers come from package references then the <Analyzer> element should be removed as part of converting to <PackageReference> to prevent duplicating the analyzer.

To repro this scenario:

1. Create a new ASP.NET project with .NET Framework
2. Add a reference to Microsoft.DotNet.UpgradeAssistant.Extensions.Default.Analyzers
3. Run try-convert
4. Observe the csproj - the <Analyzer> elements for Analyzers.dll and CodeFixes.dll are still present causing the project to see duplicate analyzers.