Remove the django_db mark from TransactionTestCase classes

pytest-django's documentation indicates that it isn't necessary, and
it turns out in Django 2.0+ that this double application of the
transaction machinations was causing the Django ContentType table to
lose its items.

awx/main/tests/functional/models/test_workflow.py

@@ -20,7 +20,6 @@ from django.test import TransactionTestCase
 from django.core.exceptions import ValidationError
 
 
-@pytest.mark.django_db
 class TestWorkflowDAGFunctional(TransactionTestCase):
     def workflow_job(self, states=['new', 'new', 'new', 'new', 'new']):
         """

awx/main/tests/functional/task_management/test_capacity.py

@@ -8,7 +8,6 @@ from awx.main.models import (
 )
 
 
-@pytest.mark.django_db
 class TestCapacityMapping(TransactionTestCase):
 
     def sample_cluster(self):

awx/main/tests/functional/test_rbac_user.py

@@ -7,7 +7,6 @@ from awx.main.access import UserAccess, RoleAccess, TeamAccess
 from awx.main.models import User, Organization, Inventory
 
 
-@pytest.mark.django_db
 class TestSysAuditorTransactional(TransactionTestCase):
     def rando(self):
         return User.objects.create(username='rando', password='rando', email='rando@com.com')