Remove one of our own jsonschema date-time formatters

We have three different ways of checking the formats of datetimes.
1. The built-in way that comes with the jsonschema package ("date-time")
2. A new way we added for broadcasts ("datetime") https://github.com/alphagov/notifications-api/commit/61a5730596ca2990851bd4e3b69c9e0c578264c9
3. An old way we defined in
   "/tests/app/public_contracts/schemas/v0/definitions.json"

In order to simplify things and make it clearer how datetimes are being
validated, this replaces the few places where we were using option 3 with option 1
instead. Option 3 was only being used to validate code that is no longer
used, the initial version of the API.

tests/app/public_contracts/schemas/v0/definitions.json

@@ -4,9 +4,5 @@
   "uuid": {
     "type": "string",
     "pattern": "^[a-fA-F0-9]{8}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{4}-[a-fA-F0-9]{12}$"
-  },
-  "datetime": {
-    "type": "string",
-    "format": "date-time"
   }
 }