Don't show any subproject tags on workboard cards

Summary: Ref T10010. This gets rid of, e.g., the "Iteration I" tag in the column for that milestone, as it is redundant with the column itself.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D15181