feat(codegen): Remove {Ser,Deser}ializer::format

This feature has never been used, and it's complicating the
implementation of #207. We could restore this functionality if there is
ever interest in it.

Closes #211.

serde/src/de/mod.rs

@@ -424,15 +424,6 @@ pub trait Deserializer {
     {
         self.deserialize(visitor)
     }
-
-    /// Specify a format string for the deserializer.
-    ///
-    /// The deserializer format is used to determine which format
-    /// specific field attributes should be used with the
-    /// deserializer.
-    fn format() -> &'static str {
-        ""
-    }
 }
 
 ///////////////////////////////////////////////////////////////////////////////

serde/src/ser/mod.rs

@@ -327,14 +327,6 @@ pub trait Serializer {
     {
         self.serialize_struct_elt(key, value)
     }
-
-    /// Specify a format string for the serializer.
-    ///
-    /// The serializer format is used to determine which format
-    /// specific field attributes should be used with the serializer.
-    fn format() -> &'static str {
-        ""
-    }
 }
 
 /// A trait that is used by a `Serialize` to iterate through a sequence.