golangci: enable unused checker on postgres code

Summary: Pull Request resolved: https://github.com/facebookincubator/ent/pull/125

Reviewed By: alexsn

Differential Revision: D18137498

fbshipit-source-id: 74cfc51e179108b818ec544c3f59655f48164193

.golangci.yml

@@ -65,10 +65,3 @@ issues:
       text: "packages.LoadSyntax is deprecated"
       linters:
         - staticcheck
-    # should be removed after postgres is enabled.
-    - path: dialect/sql/schema/postgres.go
-      linters:
-        - unused
-    - path: dialect/sql/schema/schema.go
-      linters:
-        - unused