Bug 1490525 Part 3 - Remove unused #includes in nsColumnSetFrame.cpp r=dholbert

Verified locally by s/UNIFIED_SOURCES/SOURCES/ in layout/generic/moz.build.

Differential Revision: https://phabricator.services.mozilla.com/D5636

--HG--
extra : moz-landing-system : lando

layout/generic/nsColumnSetFrame.cpp

@@ -6,8 +6,6 @@
 
 /* rendering object for css3 multi-column layout */
 
-#include "mozilla/Unused.h"
-#include "gfxPrefs.h"
 #include "nsColumnSetFrame.h"
 
 #include "mozilla/Logging.h"