Revert PR adding _listener to all template events

http://tracker.phpbb.com/browse/PHPBB3-11777
https://github.com/phpbb/phpbb3/pull/1643

The above ticket/PR adds new requirements that template events be appended with "_listener" in their file names.

There is absolutely no reason for this, and is incredibly verbose, and breaks the ease of knowing that when creating template events, you just need to name them after the event you want to inject at. (we don't need to be creating files with names like: index_body_stat_blocks_before_listener.html)

It's not enough that template events are sequestered into their own special "event" folder? There is no need to also add _listener to them.