Change PeersSerializer to SlugRelatedField

Get rid of PeersSerializer and just use SlugRelatedField,
which should be more a straightforward approach.

Other changes:
- cleanup code related to the already-removed api/v2/peers
endpoint
- add "hybrid" node type into more instance_peers test cases

awx/main/access.py

@@ -39,7 +39,6 @@ from awx.main.models import (
     Host,
     Instance,
     InstanceGroup,
-    InstanceLink,
     Inventory,
     InventorySource,
     InventoryUpdate,
@@ -2950,22 +2949,6 @@ class WorkflowApprovalTemplateAccess(BaseAccess):
         return self.model.objects.filter(workflowjobtemplatenodes__workflow_job_template__in=WorkflowJobTemplate.accessible_pk_qs(self.user, 'read_role'))
 
 
-class PeersAccess(BaseAccess):
-    model = InstanceLink
-
-    def can_add(self, data):
-        return False
-
-    def can_change(self, obj, data):
-        return False
-
-    def can_delete(self, obj):
-        return True
-
-    def can_copy(self, obj):
-        return False
-
-
 for cls in BaseAccess.__subclasses__():
     access_registry[cls.model] = cls
 access_registry[UnpartitionedJobEvent] = UnpartitionedJobEventAccess