Wordpress - How do I disable responsive images in WP 4.4?

Here are few things you could try to remove the responsive image support in 4.4:

/**
 * Disable responsive image support (test!)
 */

// Clean the up the image from wp_get_attachment_image()
add_filter( 'wp_get_attachment_image_attributes', function( $attr )
{
    if( isset( $attr['sizes'] ) )
        unset( $attr['sizes'] );

    if( isset( $attr['srcset'] ) )
        unset( $attr['srcset'] );

    return $attr;

 }, PHP_INT_MAX );

// Override the calculated image sizes
add_filter( 'wp_calculate_image_sizes', '__return_empty_array',  PHP_INT_MAX );

// Override the calculated image sources
add_filter( 'wp_calculate_image_srcset', '__return_empty_array', PHP_INT_MAX );

// Remove the reponsive stuff from the content
remove_filter( 'the_content', 'wp_make_content_images_responsive' );

but as mentioned by @cybmeta the problem may be elsewhere.

Force https on srcset

You could do some debugging with the wp_calculate_image_srcset filter and even try this quick-fix:

add_filter( 'wp_calculate_image_srcset', function( $sources )
{
    foreach( $sources as &$source )
    {
        if( isset( $source['url'] ) )
            $source['url'] = set_url_scheme( $source['url'], 'https' );
    }
    return $sources;

}, PHP_INT_MAX );

to set the url scheme to https. Another approach would be to have it schemeless //.

Check out the Codex for other set_url_scheme() options:

$source['url'] = set_url_scheme( $source['url'], null );        
$source['url'] = set_url_scheme( $source['url'], 'relative' );

But you should try to dig deeper and find the root cause.

Update:

We could bail out earlier from the wp_calculate_image_srcset() function with:

add_filter( 'wp_calculate_image_srcset_meta', '__return_empty_array' );

then using the wp_calculate_image_srcset or max_srcset_image_width filters.

Also updated according to ticket #41895, to return an empty array instead of false/null.

The simplest and cleanest way to do this is simply this:

add_filter( 'wp_calculate_image_srcset', '__return_false' );

To echo what most other folks are saying though, srcset is a good idea and is the future (best practice now), but if you need a quick fix to keep your site working, the above snippet does the job without any hacking.

source: WP Core Blog

Most likely, the reason the URLs in your srcset attributes are incorrectly showing HTTPS is because the URLs for all images are built using the value of the siteurl option in your wp_options table. If you're serving your front end over HTTPS, you should also change those values (via Settings > General).

Here's the related ticket on the WordPress issue tracking system: https://core.trac.wordpress.org/ticket/34945