Remove 'purpose' from blueprint's schema.

This has been removed from the CIP-0057 specification since validators
  are often re-used for multiple purposes (especially validators with
  arity 2). It's misleading to assign a validator a purpose since the
  purpose distinction actually happens _within_ the validator itself.

crates/aiken-lang/src/lib.rs

@@ -30,11 +30,5 @@ impl IdGenerator {
     }
 }
 
-pub const SPEND: &str = "spend";
-pub const PUBLISH: &str = "publish";
-pub const MINT: &str = "mint";
-pub const WITHDRAW: &str = "withdraw";
-pub const VALIDATOR_NAMES: [&str; 4] = [SPEND, PUBLISH, MINT, WITHDRAW];
-
 #[cfg(test)]
 mod tests;