Remove the distinction between LifetimeName::Implicit and LifetimeName::Underscore.

compiler/rustc_hir/src/intravisit.rs

@@ -496,7 +496,6 @@ pub fn walk_lifetime<'v, V: Visitor<'v>>(visitor: &mut V, lifetime: &'v Lifetime
         | LifetimeName::Param(_, ParamName::Error)
         | LifetimeName::Static
         | LifetimeName::Error
-        | LifetimeName::Implicit
         | LifetimeName::ImplicitObjectLifetimeDefault
         | LifetimeName::Underscore => {}
     }