Remove redundant definition of JoinManager from tests

The `JoinManager` class already exists in `model_utils.managers`,
with the exact same definition.

tests/models.py

@@ -5,7 +5,7 @@
 
 from model_utils import Choices
 from model_utils.fields import MonitorField, SplitField, StatusField, UUIDField
-from model_utils.managers import InheritanceManager, JoinManagerMixin, QueryManager
+from model_utils.managers import InheritanceManager, JoinManager, QueryManager
 from model_utils.models import (
     SoftDeletableModel,
     StatusModel,
@@ -418,10 +418,6 @@ class ModelWithCustomDescriptor(models.Model):
     tracker = FieldTracker(fields=['tracked_custom_field', 'tracked_regular_field'])
 
 
-class JoinManager(JoinManagerMixin, models.Manager):
-    pass
-
-
 class BoxJoinModel(models.Model):
     name = models.CharField(max_length=32)
     objects = JoinManager()