Remove warning about clj-kondo extension

[bpringe]

The warning really made the most sense right after we switched linting to come through clojure-lsp, as people would still have the clj-kondo extension installed from the previous Calva version. But by now, I think most updates should have happened, and this isn't a problem for fresh installs, unless the person had installed the clj-kondo extension manually before installing Calva.