Summary:
It was only used to pass field labels between the typechecker and
desugarer. Instead we add an extra field the RecordCon to carry this
information.

Reviewers: austin, goldfire, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1443

GHC Trac Issues: #11057