Remove OsTotal - ErlangTotal mem calculation from other_system mem

As discussed with @michaelklishin:

    We discovered that `erlang:memory(system).` can be almost as large
    as OsTotal when swapping is in effect. This means that total
    (processes + system) will be larger than the OsTotal, therefore
    OsTotal - ErlangTotal cannot be assumed to be non-negative. I think
    having some "unaccounted" memory is better than having it
    "accounted" as negative.

re #1223

[finishes #148435813]

Author: gerhard

src/rabbit_vm.erl

@@ -54,14 +54,13 @@ memory() ->
     OsTotal             = vm_memory_monitor:get_process_memory(),
 
 
-    [{total,     ErlangTotal},
-     {processes, Processes},
+    [{processes, Processes},
      {ets,       ETS},
      {atom,      Atom},
      {binary,    Bin},
      {code,      Code},
      {system,    System}] =
-        erlang:memory([total, processes, ets, atom, binary, code, system]),
+        erlang:memory([processes, ets, atom, binary, code, system]),
 
     OtherProc = Processes
         - ConnsReader - ConnsWriter - ConnsChannel - ConnsOther
@@ -97,7 +96,7 @@ memory() ->
      %% System
      {code,                Code},
      {atom,                Atom},
-     {other_system,        System - ETS - Bin - Code - Atom + (OsTotal - ErlangTotal)},
+     {other_system,        System - ETS - Bin - Code - Atom},
 
      {total,               OsTotal}
     ].