Fix an issue which could prevent blocking reviewers from being removed from revisions

Summary: Ref T11114. After evaluating typeahead tokens, we could process blocking reviewer removals incorrectly: we may get structures back.

Test Plan: Removed blocking reviewers from the web UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17163