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_feature/src/active.rs

@@ -639,9 +639,6 @@ declare_features! (
     /// Allows specifying the as-needed link modifier
     (active, native_link_modifiers_as_needed, "1.53.0", Some(81490), None),
 
-    /// Allows unnamed fields of struct and union type
-    (incomplete, unnamed_fields, "1.53.0", Some(49804), None),
-
     /// Allows qualified paths in struct expressions, struct patterns and tuple struct patterns.
     (active, more_qualified_paths, "1.54.0", Some(86935), None),