Remove django_graphiql from cookbook requirements.txt

The package is no longer required, so can be removed from:
https://github.com/graphql-python/graphene-django/blob/328cd508215c8a795b4f0a4cc613a3e13f53b40f/examples/cookbook-plain/requirements.txt#L3
https://github.com/graphql-python/graphene-django/blob/d348ec89c556f945970b5441f31ad2135948fd34/examples/cookbook/requirements.txt#L3