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<?php
// for functions.php

add_action('admin_menu', 'my_remove_sub_menus');

function my_remove_sub_menus() {
    remove_submenu_page('edit.php', 'edit-tags.php?taxonomy=category');
    remove_submenu_page('edit.php', 'edit-tags.php?taxonomy=post_tag');
}

// REMOVE POST META BOXES
function remove_my_post_metaboxes() {
  remove_meta_box( 'categorydiv','post','normal' ); // Categories Metabox
  remove_meta_box( 'tagsdiv-post_tag','post','normal' ); // Tags Metabox
}
add_action('admin_menu','remove_my_post_metaboxes');

?>