ProgressBarInner: remove misleading comment

I don't think a custom tick implementation is possible without the user
running into essentially issue console-rs#416. Coordination would be necessary
between the custom ticker and the progress bar to ensure that the custom
tick thread is joined.

src/progress_bar.rs

@@ -671,8 +671,6 @@ impl ProgressBarInner {
 }
 
 /// A weak reference to a `ProgressBar`.
-///
-/// Useful for creating custom steady tick implementations
 #[derive(Clone, Default)]
 pub struct WeakProgressBar {
     inner: Weak<Mutex<ProgressBarInner>>,