removed unused enum variant

src/error.rs

@@ -13,7 +13,6 @@ pub enum ErrorKind {
     SyntaxError(String),
     UnexpectedEof,
     UnexpectedToken(Token),
-    InvalidIdentifier,
     UnknownVariable(String),
     MeloVariable(String),
     TopLevelBreak,
@@ -52,7 +51,6 @@ impl Display for ErrorKind {
             ErrorKind::SyntaxError(desc) => write!(f, "syntax error: {}", desc),
             ErrorKind::UnexpectedEof => write!(f, "unexpected end of file"),
             ErrorKind::UnexpectedToken(token) => write!(f, "unexpected token {:?}", token),
-            ErrorKind::InvalidIdentifier => write!(f, "invalid identifier"),
             ErrorKind::UnknownVariable(name) => write!(f, "unknown identifier \"{}\"", name),
             ErrorKind::MeloVariable(name) => write!(f, "banned variable \"{}\"", name),
             ErrorKind::TopLevelBreak => write!(f, "can only `break` out of a loop"),