Revert celery to 4.4.6

There is an issue with Celery setting the BROKER_URL variable
and instead uses the default ampq broker URL. This causes problems
with running Celery so we'll delay upgrading it for now. See
 celery/celery#6285 for context.

requirements/common.txt

@@ -1,4 +1,4 @@
-celery[redis]==4.4.7
+celery[redis]==4.4.6
 Django==2.2.13
 django-compressor==2.4
 django-celery-beat==2.0.0