Bug 1674540 - Remove unused prthread.h include r=agashlin

This snuck in after I forgot to delete this upon realizing I couldn't
use nspr stuff in mozglue.

Differential Revision: https://phabricator.services.mozilla.com/D95530

mozglue/misc/PreXULSkeletonUI.cpp

@@ -17,7 +17,6 @@
 #include "mozilla/Vector.h"
 #include "mozilla/WindowsDpiAwareness.h"
 #include "mozilla/WindowsVersion.h"
-#include "prthread.h"
 
 namespace mozilla {