Remove unecessary doc comment

Author: Aaron1011

src/test/run-pass/existential_type_const.rs

@@ -6,9 +6,6 @@
 
 use std::fmt::Debug;
 
-// Type `Foo` refers to a type that implements the `Debug` trait.
-// The concrete type to which `Foo` refers is inferred from this module,
-// and this concrete type is hidden from outer modules (but not submodules).
 pub existential type Foo: Debug;
 
 const _FOO: Foo = 5;