key-func.c (guile_ssh_private_key_from_file): Remove `session' parameter

* ssh/key-func.c (guile_ssh_private_key_from_file): Remove `session'
  parameter.  All callers updated.
* doc/api-keys.texi (Keys): Update description of
  `private-key-from-file'.
* examples/echo/client.scm.in (get-prvkey): Update.
* tests/key.scm ("private-key-from-file"): Update.
* tests/client-server.scm ("userauth-public-key!, success"): Update.
* NEWS: Update.