tests: Remove unused `topic_name` parameter in tests.
tests: Remove unused `topic_name` parameter in tests.

Removes `topic_name` parameter in `test_message_flags.py`
where is being passed to a test for marking a stream as
read because it is an ignored parameter for that endpoint.

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