Within `pprDataFamInstDecl`, we were invoking `pprFamInstLHS` to
pretty-print a data family instance header, and we were passing `Just` a
kind signature to `pprFamInstLHS` to make it pretty-print the kind
signature alongside it (this is a consequence of commit
d1ef223c). But this is silly, because
then invoke `pp_data_defn`, which //also// pretty-prints the kind
signature, resulting in the kind signature being printed twice by
mistake.

This fix is simple—pass `Nothing` to `pprFamInstLHS` instead.

Test Plan: make test TEST=T14817

Reviewers: alanz, bgamari, mpickering

Reviewed By: mpickering

Subscribers: mpickering, rwbarton, thomie, carter

GHC Trac Issues: #14817

Differential Revision: https://phabricator.haskell.org/D4418