[ticket/12270] Better structured code and no debug error message

A variable (that's not essential) has been removed, two if's
has been merged to an if-else control structure. A variable
named topic_ids_list has been renamed to topic_id_list to follow
the naming standard in that file's code.
Also, the debug error has been fixed by changing the topic data
key 'username' to 'post_username' in the array_merge function
in approve_topics().

PHPBB3-12270