Its extremely difficult to remove a filter that’s added as part of an object. You have get the reference to the original object and pass that as part of the function to remove.
Luckily the filter was added at an unusual priority, so you will probably be safe removing all filters at priority 100:
remove_all_filters( 'the_content', 100 );
$content = apply_filters('the_content', '<!--subscribe2-->');
echo $content;
Just make sure that you don’t have anything else essential being added at priority 100…
If you have to find only this specific filter to remove, then I’m out of my depth. Anyone else?
Edit:
I had to figure this one out, since I’ve actually wondered for a while how to remove a filter that was passed by reference. In this case the class MnglAppController which adds the filter is initiated as a variable:
$mngl_app_controller = new MnglAppController();
So to remove it, you have to globalize that variable, and pass it as part of the remove_filter call:
global $mngl_app_controller;
remove_filter( 'the_content', array($mngl_app_controller, 'page_route'), 100 );
$content = apply_filters('the_content', '<!--subscribe2-->');
echo $content;
add_filter( 'the_content', array($mngl_app_controller, 'page_route'), 100 );