Okay, I tried this, but it's not working for me.  Here's what I did and what 
happened:

1)	I renamed ~/.trinity/share/apps/kate to ~/.trinity/share/apps/kate~
2)	I started kate (using its icon), then closed it.
	kate created a new apps directory, with an empty sessions subdirectory.
3)	I restarted kate and it still shows the sidebar with the file names.
4)	I tried creating and saving a dummy file to make kate create a session, but 
its sessions subdirectory remains empty. :-(

On 2016-12-17 09:16:20 Stefan Krusche wrote:
> After a little experimenting I found this workaround:
>
> 1) Start with pristine kate config dir
>
> cp ~/.trinity/share/apps/kate{,orig}
>
> 2) Open kate with no arguments and close it
>
> 3) Open ~/.trinity/share/apps/kate/sessions/default.session with an editor
> and change these options to "false" and save file.
> Kate-MDI-Sidebar-Visible=false
> Kate-MDI-ToolView-kate_filelist-Visible=false
>
> 4) Next time you open kate it will neither show the side bar nor the
> document list.