Remove the caret dropdown from transaction lists when no actions are available

Summary:
See PHI325. When a transaction group in Differential (or Pholio) only has an inline comment, it renders with a "V" caret but no actual dropdown menu.

This caret renders in a "disabled" color, but the color is "kinda grey". The "active" color is "kinda grey with a dab of blue". Here's what they look like today:

Just remove it.

Test Plan: Viewed one of these, no longer saw the inactive caret.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18963