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

Previously, there was a preprocessor conditional directive in
atomic_reference.c which checked for HAVE_GCC_CAS. It's gone now.
So there is no reason to waste time while compiling.

Author: alexdowad

ext/extconf.rb

@@ -37,12 +37,4 @@ 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)