node tests: Remove redundant muting tests for message lists.
node tests: Remove redundant muting tests for message lists.

This logic is already thoroughly tested in node tests for
the `MessageListData` class, and the `unmuted_messages`
method of `MessageList` only directly call the corresponding
method of the data class.

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