native: Remove generic atomic_set_return implementation

The removed implementation is the same as the generic implementation in core/atomic.c

cpu/native/atomic_cpu.c

@@ -1,36 +0,0 @@
-/**
- * Native CPU atomic.h implementation
- *
- * Copyright (C) 2013 Ludwig Ortmann <ludwig.ortmann@fu-berlin.de>
- *
- * This file is subject to the terms and conditions of the GNU Lesser
- * General Public License v2.1. See the file LICENSE in the top level
- * directory for more details.
- *
- * @ingroup arch
- * @{
- * @file
- * @author Ludwig Ortmann <ludwig.ortmann@fu-berlin.de>
- * @}
- */
-
-#include "atomic.h"
-#include "irq.h"
-#include "debug.h"
-
-unsigned int atomic_set_return(unsigned int *val, unsigned int set)
-{
-    unsigned int old_val;
-    unsigned int old_state;
-
-    DEBUG("atomic_set_return\n");
-
-    old_state = disableIRQ();
-
-    old_val = *val;
-    *val = set;
-
-    restoreIRQ(old_state);
-
-    return old_val;
-}