Remove vestigial resolveUnknown arguments from transformSortClause etc.

There's really no situation where we don't want these unknown-to-text
conversions to happen.  The alternative is failure anyway, and the one
caller that was passing "false" did so only because it expected the
case could not arise.  Might as well simplify the code.

Discussion: https://postgr.es/m/CAH2L28uwwbL9HUM-WR=hromW1Cvamkn7O-g8fPY2m=_7muJ0oA@mail.gmail.com