Remove unintended addition of a reference to the non-existant (yet) ZIP 173.
Remove unintended addition of a reference to the non-existant (yet) ZIP 173.

Signed-off-by: Daira Hopwood <cd7d43d5556a38ce626d53826fddc46adc597539@jacaranda.org>

zcash/zips,zcash/zips,zcash/zips,zcash/zips