Remove now-unneeded check for GCC CAS from extconf.rb

Author: alexdowad

ext/concurrent_ruby_ext/extconf.rb

@@ -43,14 +43,6 @@ def compiler_is_gcc
       end
     end
 
-    try_run(<<CODE,$CFLAGS) && ($defs << '-DHAVE_GCC_CAS')
-int main() {
-  int i = 1;
-  __sync_bool_compare_and_swap(&i, 1, 4);
-  return (i != 4);
-}
-CODE
-
     create_makefile(EXTENSION_NAME)
   rescue
     create_dummy_makefile