Move WordPress clean URLs to SysAdmin guide

Clean URLs in WordPress are enabled by default as of CiviCRM 5.23 so the section on enabling clean URLs is no longer a necessary step on installation.

It is however still useful information for existing sites that were installed prior to this and don't have this switched on. So, I think this information would fit better in the SysAdmin guide rather than the Installation guide.

I'm in the process of editing and updating the text for this and will submit two PRs shortly (one for each guide).