ruby_atomic.h: duplicate code

* ruby_atomic.h (ATOMIC_SIZE_CAS): remove duplicate code as
  ATOMIC_CAS().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43995 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ruby_atomic.h

@@ -16,10 +16,6 @@ typedef unsigned int rb_atomic_t;
 
 # define ATOMIC_SIZE_ADD(var, val) __atomic_fetch_add(&(var), (val), __ATOMIC_SEQ_CST)
 # define ATOMIC_SIZE_SUB(var, val) __atomic_fetch_sub(&(var), (val), __ATOMIC_SEQ_CST)
-# define ATOMIC_SIZE_CAS(var, oldval, newval) \
-({ size_t oldvaldup = (oldval); \
-   __atomic_compare_exchange_n(&(var), &oldvaldup, (newval), 0, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST); \
-   oldvaldup; })
 
 #elif defined HAVE_GCC_SYNC_BUILTINS
 /* @shyouhei hack to support atomic operations in case of gcc. Gcc