Drop Xamarin sample files

[filzrev]

Official Xamarin support is already ended on May 1, 2024

And following files appears to be no longer maintained.

• `BenchmarkDotNet.Xamarin.sln``
• samples/BenchmarkDotNet.Samples.Android
• samples/BenchmarkDotNet.Samples.iOS
• samples/BenchmarkDotNet.Samples.Forms
• docs/articles/samples/IntroXamarin.md

Is it able to drop these files?
Or If anyone is interested, migrate these files to MAUI based projects.

[adamsitnik]

Let's ask the author (#1360).

@jonathanpeppers what direction would you recommend? How much would it take to replace Xamarin with MAUI support?

[jonathanpeppers]

There is an example here:

• https://github.com/dotnet/maui/blob/main/src/Core/tests/Benchmarks.Droid/Benchmarks.Droid.csproj

It should be straightforward, and everything would work. My team has an "app building" day next week, so maybe I can do this then.