Chore: remove deprecated method (fixes docs)

[JovanVeljanoski]

Proposal: since we are soon moving to a new major version, I propose we remove some of the deprecated methods, like label_encode, especially since there are methods superseding them (ordinal_encode in this case).

This also fixes the documentation issue raised in #1061

[maartenbreddels]

I don't think we should ever remove anything if it does add maintenance burden, I suggest we just correct the message.