Remove rule requiring non-nil block-style statements to be semi-terminated

This is a subtle rule that no longer seems to be required.

src/libsyntax/ast_util.rs

@@ -38,7 +38,6 @@ pure fn stmt_id(s: stmt) -> node_id {
     alt s.node {
       stmt_decl(_, id) { id }
       stmt_expr(_, id) { id }
-      stmt_semi(_, id) { id }
     }
 }