Remove AIX workaround test_subprocess (GH-8939)


An old apparent AIX behavior workaround in test_subprocess's
test_undecodable_env is no longer needed.
(cherry picked from commit 89d79b1449750b14ded0149dcdd1e39247f2c65d)

Co-authored-by: Michael Felt <aixtools@users.noreply.github.com>
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 73b57b2..4719773 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -2228,15 +2228,9 @@
             env = os.environ.copy()
             env[key] = value
             # Use C locale to get ASCII for the locale encoding to force
-            # surrogate-escaping of \xFF in the child process; otherwise it can
-            # be decoded as-is if the default locale is latin-1.
+            # surrogate-escaping of \xFF in the child process
             env['LC_ALL'] = 'C'
-            if sys.platform.startswith("aix"):
-                # On AIX, the C locale uses the Latin1 encoding
-                decoded_value = encoded_value.decode("latin1", "surrogateescape")
-            else:
-                # On other UNIXes, the C locale uses the ASCII encoding
-                decoded_value = value
+            decoded_value = value
             stdout = subprocess.check_output(
                 [sys.executable, "-c", script],
                 env=env)