Remove edition umbrella features.

compiler/rustc_error_codes/src/error_codes/E0705.md

@@ -1,9 +1,11 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 A `#![feature]` attribute was declared for a feature that is stable in the
 current edition, but not in all editions.
 
 Erroneous code example:
 
-```rust2018,compile_fail,E0705
+```compile_fail
 #![feature(rust_2018_preview)]
 #![feature(test_2018_feature)] // error: the feature
                                // `test_2018_feature` is