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

serde_derive/src/internals/attr.rs

@@ -810,18 +810,6 @@ pub struct Variant {
     rename_all_rules: RenameAllRules,
     ser_bound: Option<Vec<syn::WherePredicate>>,
     de_bound: Option<Vec<syn::WherePredicate>>,
-    /// True if variant is a struct variant which contains a field with
-    /// `#[serde(flatten)]`.
-    ///
-    /// ```ignore
-    /// enum Enum {
-    ///     Variant {
-    ///         #[serde(flatten)]
-    ///         some_field: (),
-    ///     },
-    /// }
-    /// ```
-    has_flatten: bool,
     skip_deserializing: bool,
     skip_serializing: bool,
     other: bool,
@@ -991,7 +979,6 @@ impl Variant {
             },
             ser_bound: ser_bound.get(),
             de_bound: de_bound.get(),
-            has_flatten: false,
             skip_deserializing: skip_deserializing.get(),
             skip_serializing: skip_serializing.get(),
             other: other.get(),
@@ -1034,14 +1021,6 @@ impl Variant {
         self.de_bound.as_ref().map(|vec| &vec[..])
     }
 
-    pub fn has_flatten(&self) -> bool {
-        self.has_flatten
-    }
-
-    pub fn mark_has_flatten(&mut self) {
-        self.has_flatten = true;
-    }
-
     pub fn skip_deserializing(&self) -> bool {
         self.skip_deserializing
     }