Auth: ssh_sign, remove isinstance(message, str) check

There are no usages of ssh_sign where "message" is anything
other than a string, and it is best if we always expect it
to be a string.

Follow-up from this PR: rucio#6497

lib/rucio/common/utils.py

@@ -1279,8 +1279,7 @@ def ssh_sign(private_key, message):
     :param message: The message to sign as a string.
     :return: Base64 encoded signature as a string.
     """
-    if isinstance(message, str):
-        message = message.encode()
+    message = message.encode()
     if not EXTRA_MODULES['paramiko']:
         raise MissingModuleException('The paramiko module is not installed or faulty.')
     sio_private_key = StringIO(private_key)