Remove warning that shows on every test run. NFC (#14983)

Author: sbc100

tests/runner.py

@@ -15,7 +15,7 @@
 http://kripken.github.io/emscripten-site/docs/getting_started/test-suite.html
 """
 
-# XXX Use EMTEST_ALL_ENGINES=1 in the env to test all engines!
+# Use EMTEST_ALL_ENGINES=1 in the environment or pass --all-engined to test all engines!
 
 import argparse
 import atexit
@@ -90,9 +90,6 @@ def check_js_engines():
 
   if common.EMTEST_ALL_ENGINES:
     print('(using ALL js engines)')
-  else:
-    logger.warning('use EMTEST_ALL_ENGINES=1 in the env to run against all JS '
-                   'engines, which is slower but provides more coverage')
 
 
 def get_and_import_modules():