Remove check for __cpp_threadsafe_static_init from test_cxx11 (#27780)

These were added for eae20fe when we Emscripten was passing
the flag manually, but now the behavior is implemented and tested
upstream.

Author: dschuff

test/hello_cxx11.cpp

@@ -13,9 +13,6 @@
 #error Expected to be compiled single-threaded.
 #endif
 
-#ifdef __cpp_threadsafe_static_init
-#error single-threaded builds should not define __cpp_threadsafe_static_init
-#endif
 
 int main( int argc, const char *argv[] ) {
   printf("Hello, world!\n");