Hide comments column in WordPress backend

Try this:

add_filter("manage_edit-page_columns", "my_page_edit_columns");

function my_page_edit_columns($columns){
  unset($columns['comments']);

  return $columns;
}

If you need it for posts instead of pages, use manage_edit-post_columns instead. The same goes for any post type, really, as manage_edit-{post_type}_columns.