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_tests/tests/token.rs

@@ -301,10 +301,6 @@ impl<'a, I> ser::Serializer for Serializer<I>
         try!(key.serialize(self));
         value.serialize(self)
     }
-
-    fn format() -> &'static str {
-        "token"
-    }
 }
 
 //////////////////////////////////////////////////////////////////////////////
@@ -583,10 +579,6 @@ impl<I> de::Deserializer for Deserializer<I>
             None => Err(Error::EndOfStreamError),
         }
     }
-
-    fn format() -> &'static str {
-        "token"
-    }
 }
 
 //////////////////////////////////////////////////////////////////////////