Remove new enum variant from #50092 backport

ExpansionKind::ForeignItems was added in #49350, which is not included
in the 1.26 beta.

Author: pietroalbini

src/libsyntax/ext/base.rs

@@ -106,13 +106,6 @@ impl Annotatable {
         }
     }
 
-    pub fn expect_foreign_item(self) -> ast::ForeignItem {
-        match self {
-            Annotatable::ForeignItem(i) => i.into_inner(),
-            _ => panic!("expected foreign item")
-        }
-    }
-
     pub fn expect_stmt(self) -> ast::Stmt {
         match self {
             Annotatable::Stmt(stmt) => stmt.into_inner(),

src/libsyntax/ext/expand.rs

@@ -149,8 +149,6 @@ impl ExpansionKind {
                 Expansion::ImplItems(items.map(Annotatable::expect_impl_item).collect()),
             ExpansionKind::TraitItems =>
                 Expansion::TraitItems(items.map(Annotatable::expect_trait_item).collect()),
-            ExpansionKind::ForeignItems =>
-                Expansion::ForeignItems(items.map(Annotatable::expect_foreign_item).collect()),
             ExpansionKind::Stmts => Expansion::Stmts(items.map(Annotatable::expect_stmt).collect()),
             ExpansionKind::Expr => Expansion::Expr(
                 items.next().expect("expected exactly one expression").expect_expr()