style: [patch 3/22] reduce include dependencies in some headers

Used cppclean to help identify useless includes and removed them. This
involved erroneously included headers, but also cases where forward
declarations could have been used rather than a full include.

src/mem/multi_level_page_table.hh

@@ -38,13 +38,12 @@
 
 #include <string>
 
-#include "arch/isa_traits.hh"
 #include "arch/tlb.hh"
 #include "base/types.hh"
 #include "config/the_isa.hh"
 #include "mem/page_table.hh"
-#include "sim/serialize.hh"
-#include "sim/system.hh"
+
+class System;
 
 /**
  * This class implements an in-memory multi-level page table that can be