[GR-57998] Remove ComputedValueField and ReadableJavaField.

[graalvmbot]

Now that we have proper FieldValueTransformer, it is no longer necessary to also have ComputedValueField to do the transformations for @RecomputeFieldValue annotations. Instead, AnnotationSubstitutionProcessor now registers field value transformer too.
Only for the weird and historic final field semantics of @Alias fields, it is still necessary to have AliasField in the substitution universe. But AliasField is much simpler than the old ComputedValueField.

With ComputedValueField gone, there is also no need for ReadableJavaField.

In addition, there is also no longer a need for the Availability enum in FieldValueTransformerWithAvailabiliy. A simple boolean isAvailable method is sufficient and more flexible.

This PR should not have much impact on analysis or code quality. There is only a slight possible benefit: the field value transformer for FieldOffset transformations allows intrisification before the analysis, which can turn a few unsafe field accesses back into regular field access nodes.