Remove responsive images from feeds.

no_responsive_image_feeds.php

function no_responsive_image_feeds() {
    add_filter( 'max_srcset_image_width', function() {
       return 1;
    } );
}

add_action('rss2_head', 'no_responsive_image_feeds' );
add_action('atom_head', 'no_responsive_image_feeds' );
add_action('rss_head', 'no_responsive_image_feeds' );