Revert "Add dummy changelog entry for 3.5.1."

This reverts commit 8f6b468c.

We don't want this commit on the master branch yet.

debian/changelog

-tails (3.5.1) UNRELEASED; urgency=medium
-
-  * Dummy entry for potential emergency release.
-
- -- Tails developers <tails@boum.org>  Tue, 23 Jan 2018 01:33:25 +0100
-
 tails (3.5) unstable; urgency=medium
 
   * Security fixes