sim: Remove SimObject::setMemoryMode

Remove SimObject::setMemoryMode from the main SimObject class since it
is only valid for the System class. In addition to removing the method
from the C++ sources, this patch also removes getMemoryMode and
changeTiming from SimObject.py and updates the simulation code to call
the (get|set)MemoryMode method on the System object instead.

src/sim/sim_object.cc

@@ -163,12 +163,6 @@ SimObject::resume()
     state = Running;
 }
 
-void
-SimObject::setMemoryMode(Enums::MemoryMode new_mode)
-{
-    panic("setMemoryMode() should only be called on systems");
-}
-
 void
 SimObject::switchOut()
 {