Remove 'sign_file' from exports in c2pa.py

Removed 'sign_file' from the exported members list.

Author: tmathern

src/c2pa/c2pa.py

@@ -3735,7 +3735,6 @@ def ed25519_sign(data: bytes, private_key: str) -> bytes:
     'Builder',
     'Signer',
     'load_settings',
-    'sign_file',
     'format_embeddable',
     'version',
     'sdk_version'