remove_action( $tag, $function_to_add, $priority, $accepted_args );

This function removes a function attached to a specified action hook. This method can be used to remove default functions attached to a specific action hook and possibly replace them with a substitute.

Important: To remove a hook, the $function_to_remove and $priority arguments must match when the hook was added. This goes for both filters and actions. No warning will be given on removal failure.
via: WP Codexy

The following syntax shows an excerpt, only a part of the output you can have in your theme. They result from the standard functions, which are loaded in the head of the theme. Visible, if you search in the file wp-includes/default-filters.php for the Hook wp_head. Not all filters should be deactivated, because in most cases they are useful. But WordPress is not only as classical blog in use and therefore some functions are not necessary.

    <link rel="alternate" type="application/rss+xml" title="WP Engineer RSS Feed" href="http://wpengineer.com/feed/" />
    <link rel="alternate" type="application/atom+xml" title="WP Engineer Atom Feed" href="http://wpengineer.com/feed/atom/" />
    <link rel="pingback" href="http://wpengineer.com/blog/xmlrpc.php" />
    <link rel="EditURI" type="application/rsd+xml" title="RSD" href="http://wpengineer.com/xmlrpc.php?rsd" />
    <link rel='index' title='WP Engineer' href='http://wpengineer.com' />
    <link rel='start' title='Use WordPress 2.7 Offline' href='http://wpengineer.com/use-wordpress-27-offline/' />
    <link rel='prev' title='Recents Drafts All Authors' href='http://wpengineer.com/recents-drafts-all-authors/' />

This is an example, not a recommendation where some functions are deactivated. Check your header and turn off what you don’t need. Less markup and better loading time.

    remove_action( 'wp_head', 'feed_links_extra', 3 ); // Display the links to the extra feeds such as category feeds
    remove_action( 'wp_head', 'feed_links', 2 ); // Display the links to the general feeds: Post and Comment Feed
    remove_action( 'wp_head', 'rsd_link' ); // Display the link to the Really Simple Discovery service endpoint, EditURI link
    remove_action( 'wp_head', 'wlwmanifest_link' ); // Display the link to the Windows Live Writer manifest file.
    remove_action( 'wp_head', 'index_rel_link' ); // index link
    remove_action( 'wp_head', 'parent_post_rel_link', 10, 0 ); // prev link
    remove_action( 'wp_head', 'start_post_rel_link', 10, 0 ); // start link
    remove_action( 'wp_head', 'adjacent_posts_rel_link', 10, 0 ); // Display relational links for the posts adjacent to the current post.
    remove_action( 'wp_head', 'wp_generator' ); // Display the XHTML generator that is generated on the wp_head hook, WP version

source: wpengineer