message: attributes: remove unnecessary syn features
  This PR removes the "derive" feature, which is not used.
  The "extra-traits" flag could be eliminated, but at the cost of
  removing `Debug` implementations for a few structs.
diff: @@ -39,7 +39,7 @@ async-await = []
  [dependencies]
  proc-macro2 = "1"
  -syn = { version = "1", features = ["full", "extra-traits", "visit-mut"] }
  +syn = { version = "1", default-features = false, features = ["full", "parsing", "printing", "visit-mut", "clone-impls", "extra-traits", "proc-macro"] }
  quote = "1"
  