[Xamarin.Android.Tools.AndroidSdk] Remove hardcoded Path (#248)

Context: https://dev.azure.com/devdiv/DevDiv/_workitems/edit/2341994
Context: xamarin/androidtools@94e46bc

Remove the hardcoded path `C:\android-sdk-windows` from the list of
backup paths to check when scanning for the Android SDK.

This path was first added in xamarin/androidtools@94e46bcc, with no
explanation for *why* that was the backup path.  Perhaps that was the
convention at the time?

src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkWindows.cs

@@ -106,7 +106,6 @@ protected override IEnumerable<string> GetAllAvailableAndroidSdks ()
 					: Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.ProgramFiles), "Android", "android-sdk"),
 				Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.LocalApplicationData), "Android", "android-sdk"),
 				Path.Combine (Environment.GetFolderPath (Environment.SpecialFolder.CommonApplicationData), "Android", "android-sdk"),
-				@"C:\android-sdk-windows"
 			};
 			foreach (var basePath in paths)
 				if (Directory.Exists (basePath))