Remove-WP-Attachment-Links.php

// Disable <a> tags around images by default
function no_imagelinks_content() {
  $image_set = get_option( 'image_default_link_type' );
  
  if ($image_set !== 'none') {
    update_option('image_default_link_type', 'none');
  }
}
add_action('admin_init', 'no_imagelinks_content', 10);

// Remove them from existing posts

function attachment_image_link_remove_filter() {
  ob_start();
  $postArgs = array(
    'post_type' => 'post',
    'posts_per_page' => -1,
    'suppress_filters' => true,
    'post_status' => 'any',
    'orderby' => 'date',
    'order' => 'ASC',
  );

  $posts = get_posts($postArgs);

  foreach ($posts as $post) {
    
    $postid = $post->ID;
    $postContent = $post->post_content;

    $postModified = $post->post_modified;
    $postModifiedGMT = $post->post_modified_gmt;

    // Remove links
    $postContent =
      preg_replace(
        array('{<a(.*?)(wp-att|wp-content\/uploads)[^>]*><img}','{ wp-image-[0-9]*" /></a>}'),array('<img','" />'),
      $postContent);

    // Update Post
    $my_post = array(
      'ID' => $postid,
      'post_content' => $postContent,
      'post_modified' => $postModified,
      'post_modified_gmt' => $postModifiedGMT
    );

    $result = wp_update_post($my_post);

    if ($result > 0) {
      log_it('Success! Updated Post ID ' . $result);
    }
    else {
      log_it('Failure — Post #'.$postid);
    }
  }  
  ob_clean();
}

// add_action('wp_head', 'attachment_image_link_remove_filter');