Remove workarounds from ParameterDefaultValue class

[am11]

No description provided.

[am11]

Closing this because .NET Framework 4.8 still requires a workaround for #18844. net48 leg failed with:

Microsoft.Extensions.DependencyInjection.Specification.UnityDependencyInjectionSpecificationTests.TypeActivatorWorksWithCtorWithOptionalArgs_WithStructDefaults(createFunc: CreateInstanceFunc { Method = System.Object CreateInstanceDirectly(System.IServiceProvider, System.Type, System.Object[]), Target = null }) [FAIL]
      System.FormatException : Encountered an invalid type for a default value.
      Stack Trace:
           at System.Reflection.MdConstant.GetValue(MetadataImport scope, Int32 token, RuntimeTypeHandle fieldTypeHandle, Boolean raw)
           at System.Reflection.RuntimeParameterInfo.GetDefaultValueInternal(Boolean raw)
           at System.Reflection.RuntimeParameterInfo.get_HasDefaultValue()
        /_/src/libraries/Common/src/Extensions/ParameterDefaultValue/ParameterDefaultValue.cs(17,0): at Microsoft.Extensions.Internal.ParameterDefaultValue.TryGetDefaultValue(ParameterInfo parameter, Object& defaultValue)
        /_/src/libraries/Common/src/Extensions/ActivatorUtilities/ActivatorUtilities.cs(410,0): at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.ConstructorMatcher.CreateInstance(IServiceProvider provider)
        /_/src/libraries/Common/src/Extensions/ActivatorUtilities/ActivatorUtilities.cs(93,0): at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.CreateInstance(IServiceProvider provider, Type instanceType, Object[] parameters)
        /_/src/libraries/Microsoft.Extensions.DependencyInjection/tests/DI.Specification.Tests/ActivatorUtilitiesTests.cs(30,0): at Microsoft.Extensions.DependencyInjection.Specification.DependencyInjectionSpecificationTests.CreateInstance[T](CreateInstanceFunc func, IServiceProvider provider, Object[] args)
        /_/src/libraries/Microsoft.Extensions.DependencyInjection/tests/DI.Specification.Tests/ActivatorUtilitiesTests.cs(109,0): at Microsoft.Extensions.DependencyInjection.Specification.DependencyInjectionSpecificationTests.TypeActivatorWorksWithCtorWithOptionalArgs_WithStructDefaults(CreateInstanceFunc createFunc)