Remove fields that are dead since the removal of type ascription syntax

Since `{ ident: ident }` is a parse error, these fields are dead.

compiler/rustc_expand/src/build.rs

@@ -286,7 +286,6 @@ impl<'a> ExtCtxt<'a> {
             rules: BlockCheckMode::Default,
             span,
             tokens: None,
-            could_be_bare_literal: false,
         })
     }