[ticket/11731] Fix captcha session table queries

 The queries in the captcha garbage collection
 do a join on the session table for null keys only
 but they do not use any of the session data.

 Refactored so that they only get null keys on their
 own tables.

PHPBB3-11731
[ticket/11731] Replace acp_users.php session table queries

PHPBB3-11731
[ticket/11731] Replace acp_main.php session table queries

PHPBB3-11731
[ticket/11731] Replace functions_user session delete queries

PHPBB3-11731
[ticket/11731] Replace acp_main delete all session queries

PHPBB3-11731
[ticket/11731] Replace install_convert delete all session queries

PHPBB3-11731