Distinguishing between "refutable" and "irrefutable" patterns
(as described by the Haskell Report) in incomplete pattern errors
was more confusing than helpful. Remove references to irrefutable
patterns.

Reviewers: hvr, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie, carter

GHC Trac Issues: #14569

Differential Revision: https://phabricator.haskell.org/D4261