Un-feature gate struct variants

Struct variant field visibility is now inherited. Remove `pub` keywords
from declarations.

Closes #18641

[breaking-change]

src/test/compile-fail/deriving-span-Clone-enum-struct-variant.rs

@@ -10,7 +10,6 @@
 
 // This file was auto-generated using 'src/etc/generate-deriving-span-tests.py'
 
-#![feature(struct_variant)]
 extern crate rand;