Remove dead type definition

src/libsyntax/ast.rs

@@ -749,10 +749,6 @@ type expr = {id: node_id, callee_id: node_id, node: expr_, span: span};
 enum log_level { error, debug, other }
 // 0 = error, 1 = debug, 2 = other
 
-#[auto_serialize]
-#[auto_deserialize]
-enum alt_mode { alt_check, alt_exhaustive, }
-
 #[auto_serialize]
 #[auto_deserialize]
 enum expr_ {

src/libsyntax/parse/parser.rs

@@ -22,7 +22,7 @@ use obsolete::{
     ObsoleteWith, ObsoleteClassMethod, ObsoleteClassTraits,
     ObsoleteModeInFnType, ObsoleteByMutRefMode
 };
-use ast::{_mod, add, alt_check, alt_exhaustive, arg, arm, attribute,
+use ast::{_mod, add, arg, arm, attribute,
              bind_by_ref, bind_by_implicit_ref, bind_by_value, bind_by_move,
              bitand, bitor, bitxor, blk, blk_check_mode, bound_const,
              bound_copy, bound_send, bound_trait, bound_owned, box, by_copy,