Remove redundant restructured text entry in default Paste highlighting options

Summary: Fixes T10217. This extra entry is just a mistake because of rst + rest both being valid suffixes. We don't need both entries.

Test Plan: Edited a paste, only saw one entry in dropdown for restructured text.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10217

Differential Revision: https://secure.phabricator.com/D15114