Removed deprecated syntax for GADT constuctors.

Authored by skvadrik.

Description

Removed deprecated syntax for GADT constuctors.

Old syntax was deprecated 6 years ago in this commit
432b9c9322181a3644083e3c19b7e240d90659e7 by simonpj:"New syntax for
GADT-style record declarations, and associated refactoring" discussed
in Trac Trac #3306.

This patch removes 2 reduce/reduce conflicts in parser. Conflicting
productions were:

gadt_constr -> con_list '::' sigtype
gadt_constr -> oqtycon '{' fielddecls '}' '::' sigtype

Recursive inlining of con_list and oqtycon helped reveal the
conflict:

gadt_constr -> '(' CONSYM ')' '::' sigtype
gadt_constr -> '(' CONSYM ')' '{' fielddecls '}' '::' sigtype

between two types of GADT constructors (second form stands for
deprecated syntax).

Test Plan: make fasttest, one breakage TEST="records-fail" (parse
error instead of typecheck error due to removal of deprecated syntax).
Updated test.

Reviewers: simonmar, bgamari, austin, simonpj

Reviewed By: simonpj

Subscribers: thomie, mpickering, trofi

Differential Revision: https://phabricator.haskell.org/D1118

GHC Trac Issues: Trac #3306