management: Delete dump_messages command.
management: Delete dump_messages command.

This is basically a hacky version of `manage.py export`, implemented
primarily for development work.

Fixes part of #18898.

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