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/sim/system.hh

@@ -53,7 +53,6 @@
 
 #include "arch/isa_traits.hh"
 #include "base/loader/symtab.hh"
-#include "base/misc.hh"
 #include "base/statistics.hh"
 #include "config/the_isa.hh"
 #include "enums/MemoryMode.hh"
@@ -71,11 +70,9 @@
 #include "cpu/pc_event.hh"
 #endif
 
-class BaseCPU;
 class BaseRemoteGDB;
 class GDBListener;
 class ObjectFile;
-class Platform;
 class ThreadContext;
 
 class System : public MemObject