Remove now-unused function entityclass::changeflag()

It's now unused after I changed it so that every obj.flags access is
done directly, instead of going through this function.

desktop_version/src/Entity.h

@@ -36,8 +36,6 @@ public:
 
     void changecustomcollect(int t, int s);
 
-    void changeflag(int t, bool s);
-
     void fatal_top()
     {
         createblock(DAMAGE, -8, -8, 384, 16);