Strengthen checkbox list remarkup rule

Summary: Fixes T6657. Some other rule got stronger and started overriding this one, I think. Removes bullets from checkbox lists.

Test Plan: Looked at a checkbox list.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6657

Differential Revision: https://secure.phabricator.com/D10912