Revert "Implement Anonymous{Struct, Union} in the AST"

This reverts commit 059b68dd67.

Note that this was manually adjusted to retain some of the refactoring
introduced by commit 059b68dd67, so that it could
likewise retain the correction introduced in commit
5b4bc05fa5

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -668,7 +668,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
         // involved, so we only emit errors where there are no other parsing errors.
         gate_all!(destructuring_assignment, "destructuring assignments are unstable");
     }
-    gate_all!(unnamed_fields, "unnamed fields are not yet fully implemented");
 
     // All uses of `gate_all!` below this point were added in #65742,
     // and subsequently disabled (with the non-early gating readded).