Remove redundant gen_rev_uid.

django_etesync/migrations/0029_auto_20200312_1849.py

@@ -1,7 +1,11 @@
 # Generated by Django 3.0.3 on 2020-03-12 18:49
 
 from django.db import migrations
-from django_etesync.serializers import generate_rev_uid
+from django.utils.crypto import get_random_string
+
+
+def generate_rev_uid(length=32):
+    return get_random_string(length)
 
 
 def add_collection_main_item(apps, schema_editor):