Remove deprecated robotframework APIs

[Noordsestern]

See robotframework/robotframework#4501

SSHLibrary used to rely on deprecated API from Robot Framework. In fact, SSHLibrary has been the reason, why the API still exists. Check whether the APIs are still in use since Python2 and Jython support have been dropped.

Pekka:
You should see them if you run Python with -Wdefault or -Werror or set the equivalent environment variable.
https://docs.python.org/3/using/cmdline.html#cmdoption-W