[test] Remove check for strict warnings: in JS stdout. NFC (emscripten-core#23736)

This check was added back in 2011: acbb1e6. It looks like it was
combined with a `-s` spidermonkey flag. However, `-s` is not longer a
valid spidermonkey flag and the string "strict warnings" does not appear
in the v8 or spidermonkey source today.

Author: sbc100

test/common.py

@@ -1554,8 +1554,6 @@ def run_js(self, filename, engine=None, args=None,
       else:
         self.fail('JS subprocess failed (%s): %s (expected=%s).  Output:\n%s' % (error.cmd, error.returncode, assert_returncode, ret))
 
-    #  We should pass all strict mode checks
-    self.assertNotContained('strict warning:', ret)
     return ret
 
   def assertExists(self, filename, msg=None):