Remove warning not applicable in Rust 1.26+

Author: Havvy

src/trait-bounds.md

@@ -133,13 +133,6 @@ fn call_on_ref_zero<F>(f: F) where F: for<'a> Fn(&'a i32) {
 }
 ```
 
-<div class="warning">
-
-***Warning:*** lifetime bounds are allowed on lifetimes in a `for` binder, but
-have no effect: `for<'a, 'b: 'a>` is no different to `for<'a, 'b>`.
-
-</div>
-
 [LIFETIME_OR_LABEL]: tokens.html#lifetimes-and-loop-labels
 [_TraitPath_]: paths.html
 [`Sized`]: special-types-and-traits.html#sized