Drop unused method Organization.fetch_rhproduct_id

Before change:

    $ git grep fetch_rhproduct_id
    robottelo/api/utils.py:    prd_id = entities.Product().fetch_rhproduct_id(name=product, org_id=org_id)
    robottelo/entities.py:    def fetch_rhproduct_id(self, name):
    robottelo/entities.py:    def fetch_rhproduct_id(self, name, org_id):

After change:

    $ git grep fetch_rhproduct_id
    robottelo/api/utils.py:    prd_id = entities.Product().fetch_rhproduct_id(name=product, org_id=org_id)
    robottelo/entities.py:    def fetch_rhproduct_id(self, name, org_id):