fix aphront context bar bottom margin -- don't need it

Summary: noticed when creating a task that the "you made a task - make another" ui element had a bottom margin element which seemed out of place in the re-design. killed the margin

Test Plan: looks good to me!

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4178