Remove LinkerWorkaround.xml

[pranavkm]

We use a trimmer descriptor to work around some deficiencies in the BCL. For 5.0, this should be resolved within the BCL since these issues aren't specific to blazor.

• System.Runtime.InteropServices.JavaScript
• System.ComponentModel (ComponentModel IntrinsicTypeConverters are not linker safe runtime#38582)
• System.Text.Json.Serialization.*Converter (System.Text.Json.Serialization.JsonStringEnumConverter isn't linker-safe runtime#34449)
• Linker is trimming System.Security.Cryptography.X509Certificates even though System.Net.Http references it linker#1298

[eerhardt]

dotnet/runtime#34449 has now been fixed, so the Json Converter entries can be removed.

I have investigated the System.ComponentModel entries, and they will be fixed with dotnet/runtime#38582.