KNSCore: Avoid weird quoting/spaces in log messages

Qt handles most of that already for us and we haven't disabled
spaces/quotes explicitly.

Before:
kf.newstuff.core: Could not determine type of archive file ' "/tmp/FfmklC-Sea-otter-Bartlett-Cove-Glacier-Bay-National-Park-Alaska-Bing-4K-Wallpaper-1920x1080-10wallpaper.com.jpg" '

After:
kf.newstuff.core: Could not determine type of archive file "/tmp/jIMRhY-Hamersley-Gorge-National-Geographic-wallpaper-Wallpaper-1920x1080-10wallpaper.com.jpg"