chore(deps): drop unused graphql-iso-date (#712)

Co-authored-by: Pat <me@onpaws.com>

Author: onpaws

packages/graphile-build-pg/package.json

@@ -41,7 +41,6 @@
     "chalk": "^2.4.2",
     "debug": "^4.1.1",
     "graphile-build": "4.11.0",
-    "graphql-iso-date": "^3.6.0",
     "jsonwebtoken": "^8.5.1",
     "lodash": ">=4 <5",
     "lru-cache": ">=4 <5",