Reduce #if checks

[bartdesmet]

Remove #if checks that are always true. Flip the #if checks to encode the exceptional case rather than the common case, thus reducing the number of #defines (e.g. NO_TRACE and HAS_REMOTING are the exceptions to the rule).

NB: For the time being, I'm leaving HAS_WINRT alone. I'll review these cases separately, because they're often combined with CRIPPLED_REFLECTION which also needs some closer look. If we can deprecate UAP 10.0 at some point, a lot more #if checks can go away.

NB: The PREFER_ASYNC stuff was used to control [Obsolete] attributes on various blocking members. I'll look into the steps towards final removal of these, given they've been marked as such for a long time (will review the history to make a call; alternatively, we can make the warning stronger and start removing a lot of code by layering the blocking ones on top of the non-blocking ones using the pattern we expect users to write, e.g. .GetAwaiter().GetResult()).

[bartdesmet]

I'll leave it in place for now, though I may try to reduce the places it's needed (if any).

[clairernovotny]

@bartdesmet I just checked in .NET 5 support -- so you'll need to make sure you're using 16.8p3. That adds the .NET 5 targets, the main one does define HAS_WINRT since it's the cross platform one, and the net5.0-windows10.0.19041 does -- the latter one supports all of the Windows UI models: UWP/WPF/Winforms.