some more include file consistency changes

src/balance.cpp

@@ -20,7 +20,6 @@
 
 #include <mpi.h>
 #include <cmath>
-#include <cstdlib>
 #include <cstring>
 #include "balance.h"
 #include "atom.h"
@@ -30,7 +29,6 @@
 #include "domain.h"
 #include "force.h"
 #include "update.h"
-#include "group.h"
 #include "modify.h"
 #include "fix_store.h"
 #include "imbalance.h"
@@ -39,7 +37,6 @@
 #include "imbalance_neigh.h"
 #include "imbalance_store.h"
 #include "imbalance_var.h"
-#include "timer.h"
 #include "memory.h"
 #include "error.h"