Remove debugging output from xUnit.NET test engine

Summary: This just removes the `print "Discovered test .."` code in the xUnit.NET unit test engine.  If users need to diagnose what tests are being debugged, it can be found out with `--trace` and observing what build commands are invoked.

Test Plan: Ran `arc unit --everything` and didn't see the message any more.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7639

src/unit/engine/XUnitTestEngine.php

@@ -135,7 +135,6 @@ class XUnitTestEngine extends ArcanistBaseUnitTestEngine {
               }
 
               if (!$exists) {
-                print "Discovered test at ".$assembly."\n";
                 $results[] = array(
                   'project' => $project,
                   'assembly' => $assembly);