ssh/key-type.c (free_key_smob): Always free the SSH key on GC'ing

* ssh/key-type.h (key_data): Remove `is_to_be_freed' field.
* ssh/key-type.c (free_key_smob): Always free the SSH key on GC'ing.
* ssh/key-func.c (guile_ssh_private_key_from_file)
  (guile_ssh_public_key_from_private_key)
  (guile_ssh_public_key_from_file): Likewise.
* ssh/message-func.c (get_auth_req): Likewise.
* ssh/session-func.c (guile_ssh_get_server_public_key): Likewise.