Remove duplicated Repository.new_version() method

Required PR: pulp/pulpcore#902

[nocoverage]

related to #6463
https://pulp.plan.io/issues/6463

fixes #7844

(cherry picked from commit ef7033b)

CHANGES/7844.bugfix

@@ -0,0 +1 @@
+Fixed duplicate key error after incomplete sync task.

pulp_rpm/app/models/repository.py

@@ -6,10 +6,7 @@
 from aiohttp.web_response import Response
 from django.conf import settings
 from django.contrib.postgres.fields import JSONField
-from django.db import (
-    models,
-    transaction,
-)
+from django.db import models, transaction
 from pulpcore.plugin.download import DownloaderFactory
 from pulpcore.plugin.models import (
     Artifact,
@@ -164,6 +161,10 @@ def new_version(self, base_version=None):
 
         """
         with transaction.atomic():
+            latest_version = self.versions.latest()
+            if not latest_version.complete:
+                latest_version.delete()
+
             version = RepositoryVersion(
                 repository=self,
                 number=int(self.next_version),