I see in filters.php 

`add_action( 'wp_head', 'rest_output_link_wp_head', 10, 0 );`

Which makes me think this should do the trick to remove `rel='https://api.w.org/'`.

`remove_action( 'wp_head', 'rest_output_link_wp_head' );`

Here are all the filters in that file:

    // REST API filters.
    add_action( 'xmlrpc_rsd_apis',            'rest_output_rsd' );
    add_action( 'wp_head',                    'rest_output_link_wp_head', 10, 0 );
    add_action( 'template_redirect',          'rest_output_link_header', 11, 0 );
    add_action( 'auth_cookie_malformed',      'rest_cookie_collect_status' );
    add_action( 'auth_cookie_expired',        'rest_cookie_collect_status' );
    add_action( 'auth_cookie_bad_username',   'rest_cookie_collect_status' );
    add_action( 'auth_cookie_bad_hash',       'rest_cookie_collect_status' );
    add_action( 'auth_cookie_valid',          'rest_cookie_collect_status' );
    add_filter( 'rest_authentication_errors', 'rest_cookie_check_errors', 100 );
    
    // REST API actions.
    add_action( 'init',          'rest_api_init' );
    add_action( 'rest_api_init', 'rest_api_default_filters', 10, 1 );
    add_action( 'parse_request', 'rest_api_loaded' );