management: Delete fix_unreads command.
management: Delete fix_unreads command.

This command was part of the complex migration to introduce the
`unread_msgs` data structure as the source of truth for unreads.
Effectively, it's a migration to remove anomalies that we ran several
times before turning it into the final 0104_fix_unreads.py migration.

Fixes part of #18898.

hackerkid/zulip,rht/zulip,kou/zulip,andersk/zulip,kou/zulip,kou/zulip,kou/zulip,punchagan/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,rht/zulip,kou/zulip,rht/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,rht/zulip,rht/zulip