Undo workaround for ResXCodeFileGenerator regression

https://github.com/dotnet/wpf/issues/1495 did the following:

- Fixup Documents.resx that's part of PresenationUI with updated versions
- Bug in ResXFileCodeGenerator is failing to correctly specify types for resources, and they are being returned as object.
- Cast them correctly based on information present in Resources\Documents.resx. This is a temporary workaround and we need a real fix in the generator.

https://github.com/microsoft/msbuild/pull/4588 fixed the problem.

WPF repos can only consume the fix when it is available in Dev16.3 builds. This will be too late for .NET Core 3.0 timeframe.

We will leave the workaround in for .NET Core 3.0, but undo this workaround in .NET 5 timeframe (but after Dev16.3 bits become available).