Remove custom types marking for types which XA does not own for netcore

[marek-safar]

This is or will be handled by BCL libraries and XA tooling should try
not to interfere with more fine tuned handling done by BCL.

Simplifies [Preserve] attribute implementation and removes
old code to preserve serialization, which doesn't work
well with new .NET5/6 runtime libraries. (the old serialization
code: https://github.com/xamarin/xamarin-android/blob/master/src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/ApplyPreserveAttribute.cs#L57-L82)

The serialization is currently broken
and will be fixed in runtime libs. Part of CustomLinkDescriptionPreserve
test is thus disabled.

Also the attribute removal is now handled in illink.

The serialization fix leads to great reduction of assemblies size in simple
XA test app by cca 1MB, apkdiff before/after:

Summary:
  -   1,199,411 Assemblies -55.66% (of 2,154,984)
  -   1,206,333 Package size difference -13.37% (of 9,024,291)

[jonathanpeppers]

If this test fails, you might check for a different assembly like System.Linq.dll. I think System.Private.CoreLib.dll can be in a sub-directory like armeabi-v7a or x86.

[marek-safar]

System.Private.CoreLib.dll is probably only reliable marker. I hope System.Linq.dll will be gone too next week ;-)

[jonpryor]

@radekdoulik: please address the merge conflicts.

Additionally, you had mentioned that this change would "implicitly" break apps requiring serialization, e.g. dotnet/runtime#45559

Please write up a description/explanation for the non-obvious relation here.