Remove IWA (Integrated Windows Authentication) auth mode

[dggsax]

Summary

IWA should be dropped from the Windows auth modes. With broker (WAM) support mature on Windows and now macOS broker support added, IWA is no longer needed. Removing it would unify the auth mode enum between platforms.

Context

Raised by @kyle-rader-msft in PR #453 review:

IWA should really be dropped from Windows at this point, which means finally the auth modes between mac and windows are actually the same. Should be a follow up change

Changes needed

• Remove AuthMode.IWA from the [Flags] enum
• Remove IntegratedWindowsAuthentication auth flow class
• Remove IWA from AuthFlowFactory.Create()
• Remove IsIWA() extension method
• Remove iwa from AuthModeAllowedValues
• Collapse #if PlatformWindows / #else blocks in AuthMode.cs (modes become identical across platforms)
• Update tests and docs