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/api/views/__init__.py

@@ -4349,17 +4349,3 @@ class WorkflowApprovalDeny(RetrieveAPIView):
             return Response({"error": _("This workflow step has already been approved or denied.")}, status=status.HTTP_400_BAD_REQUEST)
         obj.deny(request)
         return Response(status=status.HTTP_204_NO_CONTENT)
-
-
-class PeersList(ListAPIView):
-    name = _("Peers")
-    model = models.InstanceLink
-    serializer_class = serializers.InstanceLinkSerializer
-    search_fields = ('source', 'target', 'link_state')
-
-
-class PeersDetail(RetrieveAPIView):
-    name = _("Peers Detail")
-    always_allow_superuser = True
-    model = models.InstanceLink
-    serializer_class = serializers.InstanceLinkSerializer