makeParamHelpList() broken due to R help page update

[pat-s]

@mb706 Pinging you, since you implemented this one in 2017.

Specifically, makeParamHelpList does not work anymore as the returned list in tab is empty (see link) - I suspect the HTML structure in the R help pages got updated (I don't see the element //table[@summary='R argblock'] anymore in the html output of a help page).
See also https://github.com/mlr-org/mlr/runs/7383000156?check_suite_focus=true#step:21:456.

I've digged in a bit but never fiddled around with {XML} so far and it would take me probably too much time to fix this for a quite low return.

Do you want to fix this? If not, I'd remove the logic and related tests.

[mb706]

Back from my vacation, I will look into this