Simplify implementation of flattened internally tagged enum

The fact that the tag entry is consumed was only observable if there is
a later flattened map field, at which point the behavior is already
confusing anyway.

    #[derive(Deserialize)]
    struct Example {
        #[serde(flatten)]
        a: InternallyTagged,
        #[serde(flatten)]
        rest: BTreeMap<String, Value>,
    }

Before this simplification the map would receive all the fields of the
internally tagged enum but not its tag, after it receives all the fields
including the tag.

serde_derive/src/de.rs

@@ -1195,8 +1195,7 @@ fn deserialize_internally_tagged_enum(
 
         let __tagged = try!(_serde::Deserializer::private_deserialize_internally_tagged_enum(
             __deserializer,
-            _serde::private::de::TaggedContentVisitor::<__Field>::new(#tag),
-            #tag));
+            _serde::private::de::TaggedContentVisitor::<__Field>::new(#tag)));
 
         match __tagged.tag {
             #(#variant_arms)*