Remove deprecated expand() feature from docs.

[MichelDiz]

Documentation

Context https://discuss.dgraph.io/t/how-to-pass-predicates-stored-in-a-variable-to-expand-function/5639

Section to be reviewed https://docs.dgraph.io/query-language/#expand-predicates
Old context https://docs.dgraph.io/query-language/#expand-predicates

As this functionality was removed in favor of the new Type System. We need to check if there's still any reason to keep it there in docs.