Remove now unused double_braced_* symbols

Author: marmeladema

compiler/rustc_span/src/symbol.rs

@@ -441,13 +441,6 @@ symbols! {
         document_private_items,
         dotdot_in_tuple_patterns,
         dotdoteq_in_patterns,
-        double_braced_closure: "{{closure}}",
-        double_braced_constant: "{{constant}}",
-        double_braced_constructor: "{{constructor}}",
-        double_braced_crate: "{{crate}}",
-        double_braced_impl: "{{impl}}",
-        double_braced_misc: "{{misc}}",
-        double_braced_opaque: "{{opaque}}",
         drop,
         drop_in_place,
         drop_types_in_const,