Remove obsolete f= from viewtopic URL

The f= parameter in a viewtopic URL is not needed as all viewtopic URLs work without it and as the canonical URL format does not have it.

The f= parameter was required in order to display a user / visitor in the forum's "who is online" list:

http://area51.phpbb.com/phpBB/viewtopic.php?p=264315#p264315

Because of that, visitors coming from canonical URLs (without the f=) were not displayed in the list. This was known as the "ghost users" bug which has now been fixed:

https://github.com/phpbb/phpbb/pull/5012

Thus, the f= parameter is now obsolete and can be removed - and as a result we can have simpler looking and prettier URLs.