[mono] Put WeakAttribute support under an ifdef (#66213)

[mono] Put WeakAttribute support under an ifdef (#66213)

This code has been effectively dead in .NET 6 on all platforms since
System.WeakAttribute is not in CoreLib.

Not everything is removed:

- MonoClass:has_weak_fields bit is still present (and always 0).

- the AOT compiler still emits a (size 0) weak_field_indexes symbol (but the
  AOT runtime reader code is under an ifdef).  Likewise the AOT table
  definitions, etc are still there.  It seemed unnecessary to perturb the AOT
  format.