sim: remove unused MemoryModeStrings array

The System object has a static MemoryModeStrings array
that's (1) unused and (2) redundant, since there's an
auto-generated version in the Enums namespace.  No
point in leaving it in.

src/sim/system.cc

@@ -462,9 +462,6 @@ System::getMasterName(MasterID master_id)
     return masterIds[master_id];
 }
 
-const char *System::MemoryModeStrings[4] = {"invalid", "atomic", "timing",
-                                            "atomic_noncaching"};
-
 System *
 SystemParams::create()
 {

src/sim/system.hh

@@ -121,8 +121,6 @@ class System : public MemObject
     BaseMasterPort& getMasterPort(const std::string &if_name,
                                   PortID idx = InvalidPortID);
 
-    static const char *MemoryModeStrings[4];
-
     /** @{ */
     /**
      * Is the system in atomic mode?