settings: Remove ANONYMOUS_USER_ID.
settings: Remove ANONYMOUS_USER_ID.

This finishes up #5498 removing the forgotten variable added when we
introduced the later removed django-guardian.

zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip