How Do I Unhook This Parent Theme Function?

wp_loaded would fire too early – you can’t remove a hook before it’s been added.

I’d try adding your own hook with a higher priority, at the same level as the parent’s hook. That is, not inside an action.

add_filter( 'preprocess_comment' , 'my_preprocess_comment_function', 5 );

Then within your function, remove the parent function:

function my_preprocess_comment_function( $commentdata ) {
    remove_filter( 'preprocess_comment', 'preprocess_comment_remove_url' );
// Do anything else you need to do
return $commentdata;
}