Un-feature gate struct variants

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

Closes #18641

[breaking-change]
auto merge of #18994 : sfackler/rust/struct-variants-pt2, r=jakub-

Struct variant field visibility is now inherited. Remove `pub` keywords
from declarations.

Closes #18641

[breaking-change]

r? @alexcrichton