You can use the_content_feed
hook for that. It is used to filter the content of the post after it is retrieved from the database and filtered by “the_content” and before it is sent to RSS reader (or browser).
So here’s the code:
add_filter( 'the_content_feed', function ( $content ) {
$content = str_replace( '<div class="well"> <a class="btn" href="https://wordpress.stackexchange.com/questions/330141/foo.com">foo</a></div>', '', $content );
return $content;
} );