mem: Avoid explicitly zeroing the memory backing store

This patch removes the explicit memset as it is redundant and causes
the simulator to touch the entire space, forcing the host system to
allocate the pages.

Anonymous pages are mapped on the first access, and the page-fault
handler is responsible for zeroing them. Thus, the pages are still
zeroed, but we avoid touching the entire allocated space which enables
us to use much larger memory sizes as long as not all the memory is
actually used.

src/mem/AbstractMemory.py

@@ -48,7 +48,6 @@ class AbstractMemory(MemObject):
     cxx_header = "mem/abstract_mem.hh"
     range = Param.AddrRange(AddrRange('128MB'), "Address range")
     null = Param.Bool(False, "Do not store data, always return zero")
-    zero = Param.Bool(False, "Initialize memory with zeros")
 
     # All memories are passed to the global physical memory, and
     # certain memories may be excluded from the global address map,