Remove last remnants of PAPERLESS_SHARED_SECRET

src/documents/mail.py

@@ -42,10 +42,7 @@ class Message(Loggable):
     and n attachments, and that we don't care about the message body.
     """
 
-    SECRET = os.getenv(
-        "PAPERLESS_EMAIL_SECRET",
-        os.getenv("PAPERLESS_SHARED_SECRET")  # TODO: Remove after 2017/09
-    )
+    SECRET = os.getenv("PAPERLESS_EMAIL_SECRET")
 
     def __init__(self, data, group=None):
         """