post #6 https://www.ads-software.com/support/topic/is-there-a-way-to-disable-the-new-media-manager?replies=30
add_action('admin_head', 'remove_media_buttons');
function remove_media_buttons(){
remove_action('media_buttons','media_buttons');
add_action('media_buttons', 'old_media_buttons');
}
function old_media_buttons($editor_id = 'content') {
$context = apply_filters('media_buttons_context', __('Upload/Insert %s'));
$img = '<img src="' . esc_url( admin_url( 'images/media-button.png?ver=20111005' ) ) . '" width="15" height="15" />';
echo '<a href="' . esc_url( get_upload_iframe_src() ) . '" class="thickbox add_media" id="' . esc_attr( $editor_id ) . '-add_media" title="' . esc_attr__( 'Add Media' ) . '" onclick="return false;">' . sprintf( $context, $img ) . '</a>';
}
