Remove unused variable

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

poezio_omemo/__init__.py

@@ -153,7 +153,7 @@ class Plugin(E2EEPlugin):
                     'I can\'t decrypt this message as it '
                     'is not encrypted for me.'
                 )
-        except (NoAvailableSession,) as exn:
+        except (NoAvailableSession,):
             # We received a message from that contained a session that we
             # don't know about (deleted session storage, etc.). We can't
             # decrypt the message, and it's going to be lost.