[PHPBB3-9606] Drop redundant SQL query for unreads fetching

Currently we have redundant SQL query in search.php in order to carry out search for unread posts:

				if (!empty($unread_list))

					$sql = 'SELECT t.topic_id

						FROM ' . TOPICS_TABLE . ' t

						WHERE ' . $db->sql_in_set('t.topic_id', array_keys($unread_list)) . "

						$sql_sort";

					$field = 'topic_id';

It should be dropped to speedup the things.

is duplicated by

PHPBB3-9636 Search for unread posts uses message_body template instead of search_results template when there's no unread posts to display.