Remove SSlibSigner mention in docstring

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Author: Lukas Puehringer

tuf/api/metadata.py

@@ -351,9 +351,7 @@ def sign(
 
         Args:
             signer: A ``securesystemslib.signer.Signer`` object that provides a
-            private key and signing implementation to generate the signature. A
-            standard implementation is available in
-            ``securesystemslib.signer.SSlibSigner``.
+                signing implementation to generate the signature.
             append: ``True`` if the signature should be appended to
                 the list of signatures or replace any existing signatures. The
                 default behavior is to replace signatures.