Remove AIX workaround test_subprocess (GH-8939)

An old apparent AIX behavior workaround in test_subprocess's
test_undecodable_env is no longer needed.

Lib/test/test_subprocess.py

@@ -2228,15 +2228,9 @@ class POSIXProcessTestCase(BaseTestCase):
             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
+            # surrogate-escaping of \xFF in the child process
-            # be decoded as-is if the default locale is latin-1.
             env['LC_ALL'] = 'C'
-            if sys.platform.startswith("aix"):
+            decoded_value = value
-                # 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
             stdout = subprocess.check_output(
                 [sys.executable, "-c", script],
                 env=env)

Misc/NEWS.d/next/Tests/2018-08-26-13-12-34.bpo-11193.H8fCGa.rst

@@ -0,0 +1 @@
+Remove special condition for AIX in `test_subprocess.test_undecodable_env`