anyhow: Remove ReadUsernameError::IoError

As far as I understand, the example with the `anyhow` example
does not use its special `IoError` anymore, unlike the previous
example with just `thiserror`.

I think this should be made clear. I considered just adding a
`// Now unused` comment, but I think ti's clearer to just remove
the unused code.

src/error-handling/error-contexts.md

@@ -11,8 +11,6 @@ use anyhow::{Context, Result};
 
 #[derive(Error, Debug)]
 enum ReadUsernameError {
-    #[error("Could not read: {0}")]
-    IoError(#[from] io::Error),
     #[error("Found no username in {0}")]
     EmptyUsername(String),
 }