Don't say the board must be empty to be modified

It is possible to empty a full board and have modifications that
should be saved.

Fixes https://gitlab.com/kicad/code/kicad/issues/3739

pcbnew/pcb_edit_frame.cpp

@@ -436,7 +436,7 @@ void PCB_EDIT_FRAME::SetPageSettings( const PAGE_INFO& aPageSettings )
 
 bool PCB_EDIT_FRAME::IsContentModified()
 {
-    return GetScreen() && GetScreen()->IsModify() && GetBoard() && !GetBoard()->IsEmpty();
+    return GetScreen() && GetScreen()->IsModify();
 }