Clean up remaining Xamarin monikers/parameters

We deleted the Xamarin assemblies and re-ran the CI (mdoc) in #9609. However, not all of the monikers and Xamarin-specific parameters were removed, so we need to do that work manually.

Related to https://github.com/dotnet/dotnet-api-docs/issues/9471.

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[Associated WorkItem - 232979](https://dev.azure.com/msft-skilling/Content/_workitems/edit/232979)