* Don't validate the TargetFrameworkIdentifier, instead pass it along to bgen
(which will validate it). This means less validation updates when something
changes.
* For Xamarin.Mac's GenerateCommandLineCommands' implementation:
* We'll always have a bgen.exe, so the code for what to do if bgen isn't
there can be removed.
* There's no need to compute the 'isMobile' value, because now it's not used
anymore. So we can remove the first if block in the method completely.
* There's no need to add the -stdlib flag, because the base implementation
in BTouchTaskBase already adds it.
* Now there's nothing left in the method but calling base, so the entire
override can be removed.
* This also removes the need for the FrameworkRoot parameter to the task,
so remove that too.
* For Xamarin.Mac's GetTargetFrameworkArgument's implementation:
* Move the TargetFramework logic to detect the different Xamarin.Mac
variations to the targets file, so that it can be reused by other tasks
and targets.
* This means we don't need an overridable function to get the target
framework argument, so just remove inline the entire virtual method.
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