Use hello_world.c rather than hello_world.cpp for metadce test. NFC (#20957)

I think this file was originally copied from `test/hello_world.cpp`
before we had the pure C version in place.

Author: sbc100

test/other/metadce/hello_world.cpp renamed to test/other/metadce/hello_world.c

@@ -5,8 +5,6 @@
 
 #include <stdio.h>
 
-class Test {}; // This will fail in C mode
-
 int main() {
   printf("hello, world!\n");
   return 0;

test/test_other.py

@@ -8330,7 +8330,7 @@ def test_metadce_cxx(self, *args):
     'wasmfs': (['-O3', '-sWASMFS'],        [], []), # noqa
   })
   def test_metadce_hello(self, *args):
-    self.run_metadce_test('hello_world.cpp', *args)
+    self.run_metadce_test('hello_world.c', *args)
 
   @parameterized({
     'O3':                 ('mem.c', ['-O3'],