Replace MapRefDeserializer with value::MapDeserializer

Although they are slightly different, this difference is irrelevant:
- MapDeserializer has a specialization for deserialize_seq and deserialize_tuple, but
  only MapRefDeserializer::deserialize_any is used by the code which is almost the same
- MapDeserializer checks that map was consumed after visit_map, but MapRefDeserializer
  does not. Actually, each derived implementation consumes map and each manual implementation
  also should consume it

Also, MapDeserializer already used when value deserialized from ContentRefDeserializer
directly and MapRefDeserializer was only used to deserialize Struct variants of enums.
There are no reasons why the behavior should be different in those two cases

serde/src/private/de.rs

@@ -2213,9 +2213,7 @@ mod content {
             match self.value {
                 // Covered by tests/test_enum_untagged.rs
                 //      newtype_enum::struct_from_map
-                Some(Content::Map(v)) => {
-                    de::Deserializer::deserialize_any(MapRefDeserializer::new(v), visitor)
-                }
+                Some(Content::Map(v)) => visit_content_map_ref(v, visitor),
                 // Covered by tests/test_enum_untagged.rs
                 //      newtype_enum::struct_from_seq
                 //      newtype_enum::empty_struct_from_seq
@@ -2232,83 +2230,6 @@ mod content {
         }
     }
 
-    struct MapRefDeserializer<'a, 'de: 'a, E>
-    where
-        E: de::Error,
-    {
-        iter: <&'a [(Content<'de>, Content<'de>)] as IntoIterator>::IntoIter,
-        value: Option<&'a Content<'de>>,
-        err: PhantomData<E>,
-    }
-
-    impl<'a, 'de, E> MapRefDeserializer<'a, 'de, E>
-    where
-        E: de::Error,
-    {
-        fn new(map: &'a [(Content<'de>, Content<'de>)]) -> Self {
-            MapRefDeserializer {
-                iter: map.iter(),
-                value: None,
-                err: PhantomData,
-            }
-        }
-    }
-
-    impl<'de, 'a, E> de::MapAccess<'de> for MapRefDeserializer<'a, 'de, E>
-    where
-        E: de::Error,
-    {
-        type Error = E;
-
-        fn next_key_seed<T>(&mut self, seed: T) -> Result<Option<T::Value>, Self::Error>
-        where
-            T: de::DeserializeSeed<'de>,
-        {
-            match self.iter.next() {
-                Some((key, value)) => {
-                    self.value = Some(value);
-                    seed.deserialize(ContentRefDeserializer::new(key)).map(Some)
-                }
-                None => Ok(None),
-            }
-        }
-
-        fn next_value_seed<T>(&mut self, seed: T) -> Result<T::Value, Self::Error>
-        where
-            T: de::DeserializeSeed<'de>,
-        {
-            match self.value.take() {
-                Some(value) => seed.deserialize(ContentRefDeserializer::new(value)),
-                None => Err(de::Error::custom("value is missing")),
-            }
-        }
-
-        fn size_hint(&self) -> Option<usize> {
-            size_hint::from_bounds(&self.iter)
-        }
-    }
-
-    impl<'de, 'a, E> de::Deserializer<'de> for MapRefDeserializer<'a, 'de, E>
-    where
-        E: de::Error,
-    {
-        type Error = E;
-
-        #[inline]
-        fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
-        where
-            V: de::Visitor<'de>,
-        {
-            visitor.visit_map(self)
-        }
-
-        forward_to_deserialize_any! {
-            bool i8 i16 i32 i64 i128 u8 u16 u32 u64 u128 f32 f64 char str string
-            bytes byte_buf option unit unit_struct newtype_struct seq tuple
-            tuple_struct map struct enum identifier ignored_any
-        }
-    }
-
     impl<'de, E> de::IntoDeserializer<'de, E> for ContentDeserializer<'de, E>
     where
         E: de::Error,