Correctly calculate has_flatten attribute in all cases for deserialization

Consequence: `FlattenSkipDeserializing[DenyUnknown]`
- does not collect data in Field, because do not read them anyway
- gets `deserialize_in_place` method
- gets ability to deserialize from sequence (visit_seq method)
- uses `deserialize_struct` instead of `deserialize_map`

serde_derive/src/internals/ast.rs

@@ -85,7 +85,6 @@ impl<'a> Container<'a> {
                     for field in &mut variant.fields {
                         if field.attrs.flatten() {
                             has_flatten = true;
-                            variant.attrs.mark_has_flatten();
                         }
                         field.attrs.rename_by_rules(
                             variant