dev: Remove unused system pointer in the Platform base class

The Platform base class contains a pointer to an instance of the
System which is never initialized. This can lead to subtle bugs since
some architecture-specific platform implementations contain their own
system pointer which is normally used. However, if the platform is
accessed through a pointer to its base class, the dangling pointer
will be used instead.

src/dev/platform.hh

@@ -55,9 +55,6 @@ class Platform : public SimObject
     /** Pointer to the interrupt controller */
     IntrControl *intrctrl;
 
-    /** Pointer to the system for info about the memory system. */
-    System *system;
-
   public:
     typedef PlatformParams Params;
     Platform(const Params *p);